fix(agent): MCP 读日志/诊断崩溃改走 session_index::event_file（D2 第三处读取点）

M212（M211 复审观察 7 确认的真 bug）：

- `agent/ports/events.rs::event_path` 仍在 `root.join("game").join(&entry.log_path)`，
  而 `log_path` 已经是**相对会话根**的 `game/<id>.jsonl` ⇒ 对每个真实会话解析成
  `game/game/<id>.jsonl`。后果是 `ull agent` / MCP 的 read_game_log 与 diagnose_crash
  两个工具对真实会话全报「读会话事件流失败」——内置 AI Agent 的看家本领是坏的。
  现在解析只走 M211 建立的唯一真相源 `session_index::event_file`（新旧两个根都走它）。
- 夹具：`ports/session.rs`、`ports/diagnose.rs` 的索引夹具此前写裸文件名
  `20261029T124501-9f3c2a.jsonl`，与生产差一层 `game/`，所以这个 bug 在测试里永远看不见。
  改成 `session_index::log_path_for(id)` + `session_index::event_file` 落盘。
- 用例：`ports/events.rs` 新增三条（解析结果 = 写入侧落盘的那一个 / 文件缺失时指向的仍是
  生产形状 / 旧根回落走同一套解析）；`cli/tests/agent_mcp_stdio_test.rs` 新增一条真机
  MCP 用例（起 `ull agent serve --stdio`，对一份生产形状的会话调 list_game_sessions →
  read_game_log → diagnose_crash）。旧实现下两条路径都红，红的是
  `...\logs\game\game\20261030T101500-abcdef.jsonl`。

crates/cli/src/commands/agent/ports/diagnose.rs

@@ -128,13 +128,17 @@ impl DiagnosePort for CoreDiagnose {
 mod tests {
     use super::*;
     use serde_json::json;
+    use unlucky_core::launch::session_index;
 
+    /// 与生产写入同形（M212/D2）：`log_path` 相对会话根，事件流落在 `<根>/game/<id>.jsonl`。
     fn seed(root: &std::path::Path, stderr_line: &str) {
-        let game = root.join("game");
-        std::fs::create_dir_all(&game).expect("建目录");
+        let sid = "20261029T124501-9f3c2a";
+        let log_path = session_index::log_path_for(sid);
+        let index = session_index::index_path(root);
+        std::fs::create_dir_all(index.parent().expect("索引父目录")).expect("建目录");
         let entry = json!({
             "v": 1,
-            "session_id": "20261029T124501-9f3c2a",
+            "session_id": sid,
             "instance_id": "inst-1",
             "instance_name": "1.20.1 Forge",
             "game_version": "1.20.1",
@@ -146,18 +150,18 @@ mod tests {
             "silent_failure": false,
             "window_detected": false,
             "event_count": 1,
-            "log_path": "20261029T124501-9f3c2a.jsonl",
+            "log_path": log_path.clone(),
             "crash_found": true,
             "diagnosis_top": null
         });
-        std::fs::write(game.join("index.jsonl"), format!("{entry}\n")).expect("写索引");
+        std::fs::write(index, format!("{entry}\n")).expect("写索引");
         let event = json!({
             "seq": 0, "ts": "2026-10-29T12:45:01.123Z", "t_ms": 0,
             "kind": "stderr",
             "payload": { "line": stderr_line, "level": "error" }
         });
         std::fs::write(
-            game.join("20261029T124501-9f3c2a.jsonl"),
+            session_index::event_file(root, &log_path),
             format!("{event}\n"),
         )
         .expect("写事件");

crates/cli/src/commands/agent/ports/events.rs

@@ -120,15 +120,21 @@ impl SessionSource {
     }
 
     /// 事件流文件路径（新根没有时回落到旧根）。
+    ///
+    /// 解析只走 [`session_index::event_file`] 一处（M212/D2）：索引里的 `log_path` 已经是
+    /// **相对会话根**的 `game/<id>.jsonl`，这里再拼一层 `game/` 就会拼成
+    /// `game/game/<id>.jsonl` —— `ull agent` / MCP 的「读游戏日志」「诊断崩溃」两个工具
+    /// 因此对每个真实会话都报「读会话事件流失败」，夹具写裸文件名时这个 bug 测不出来。
     #[must_use]
     pub fn event_path(&self, entry: &SessionIndexEntry) -> PathBuf {
         for root in self.roots() {
-            let path = root.join("game").join(&entry.log_path);
+            let path = session_index::event_file(root, &entry.log_path);
             if path.is_file() {
                 return path;
             }
         }
-        self.root.join("game").join(&entry.log_path)
+        // 两处都没有：返回**新根**的路径，调用方据此说出「该去哪找」。
+        session_index::event_file(&self.root, &entry.log_path)
     }
 
     /// 事件里的 stdout / stderr 文本（诊断与日志阅读共用）。
@@ -174,3 +180,100 @@ pub fn tail_join(lines: &[String], tail: usize) -> String {
 pub fn index_exists(root: &Path) -> bool {
     session_index::index_path(root).is_file()
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    const SID: &str = "20261029T124501-9f3c2a";
+
+    fn entry(log_path: &str) -> SessionIndexEntry {
+        serde_json::from_value(json!({
+            "v": 1,
+            "session_id": SID,
+            "instance_id": "inst-1",
+            "instance_name": "1.20.1 Forge",
+            "game_version": "1.20.1",
+            "started_at": "2026-10-29T12:45:01.123Z",
+            "ended_at": "2026-10-29T12:46:31.123Z",
+            "duration_ms": 90000,
+            "exit_code": 1,
+            "exit_type": "application_error",
+            "silent_failure": false,
+            "window_detected": true,
+            "event_count": 1,
+            "log_path": log_path,
+            "crash_found": false,
+            "diagnosis_top": null
+        }))
+        .expect("索引条目")
+    }
+
+    fn temp_root(tag: &str) -> PathBuf {
+        let root =
+            std::env::temp_dir().join(format!("ull-agent-events-{tag}-{}", std::process::id()));
+        let _ = std::fs::remove_dir_all(&root);
+        std::fs::create_dir_all(root.join("game")).expect("建临时根");
+        root
+    }
+
+    /// 抓得住：新根解析出的路径**正好**是写入侧落盘的那一个（`<根>/game/<id>.jsonl`）。
+    /// 抓不住：旧根回落（下一条用例单独守）与事件内容解析（`ports::session` 的用例守）。
+    #[test]
+    fn event_path_matches_what_the_writer_wrote() {
+        let root = temp_root("primary");
+        let log_path = session_index::log_path_for(SID);
+        let written = session_index::event_file(&root, &log_path);
+        std::fs::write(&written, "{\"seq\":0}\n").expect("写事件流");
+
+        let source = SessionSource::new(root.clone());
+        let resolved = source.event_path(&entry(&log_path));
+
+        assert_eq!(resolved, root.join("game").join(format!("{SID}.jsonl")));
+        assert!(
+            resolved.is_file(),
+            "解析出的路径应真的是写入侧那份：{resolved:?}"
+        );
+
+        let _ = std::fs::remove_dir_all(&root);
+    }
+
+    /// 文件暂时不在时，返回的「该去哪找」也必须是生产形状，不能多一层 `game/`。
+    #[test]
+    fn missing_event_file_still_points_at_the_production_shape() {
+        let root = temp_root("missing");
+        let log_path = session_index::log_path_for(SID);
+
+        let source = SessionSource::new(root.clone());
+        let resolved = source.event_path(&entry(&log_path));
+
+        assert_eq!(resolved, session_index::event_file(&root, &log_path));
+        assert_eq!(resolved, root.join("game").join(format!("{SID}.jsonl")));
+
+        let _ = std::fs::remove_dir_all(&root);
+    }
+
+    /// 旧根（`data/agent/sessions`，M188 前的生产落点）里的存量会话仍要读得回来。
+    #[test]
+    fn legacy_root_fallback_uses_the_same_resolution() {
+        let primary = temp_root("legacy-primary");
+        let legacy = temp_root("legacy-old");
+        let log_path = session_index::log_path_for(SID);
+        let legacy_file = session_index::event_file(&legacy, &log_path);
+        std::fs::create_dir_all(legacy_file.parent().expect("父目录")).expect("建 game/");
+        std::fs::write(&legacy_file, "{\"seq\":0}\n").expect("写旧根事件流");
+
+        let source = SessionSource::new(primary.clone()).with_legacy(legacy.clone());
+        let resolved = source.event_path(&entry(&log_path));
+
+        assert_eq!(resolved, legacy_file);
+        assert!(
+            resolved.is_file(),
+            "旧根里的存量会话应读得回来：{resolved:?}"
+        );
+
+        let _ = std::fs::remove_dir_all(&primary);
+        let _ = std::fs::remove_dir_all(&legacy);
+    }
+}

crates/cli/src/commands/agent/ports/session.rs

@@ -142,10 +142,15 @@ pub fn log_event_json(event: &LogEvent) -> Value {
 mod tests {
     use super::*;
     use std::path::Path;
+    use unlucky_core::launch::session_index;
 
+    /// 生产写入侧真会写出的形状（M212/D2）：`log_path` 相对**会话根**，事件流真落点
+    /// `<根>/game/<id>.jsonl`。夹具以前写裸文件名、把事件流也放在 `game/` 下，
+    /// 与生产只差一层 `game/` —— 正是这一层让「读取侧多 join 一层」永远测不出来。
     fn seed_session(root: &Path) {
-        let game_dir = root.join("game");
-        std::fs::create_dir_all(&game_dir).expect("建目录");
+        let log_path = session_index::log_path_for("20261029T124501-9f3c2a");
+        let index = session_index::index_path(root);
+        std::fs::create_dir_all(index.parent().expect("索引父目录")).expect("建目录");
         let entry = json!({
             "v": 1,
             "session_id": "20261029T124501-9f3c2a",
@@ -160,11 +165,11 @@ mod tests {
             "silent_failure": false,
             "window_detected": true,
             "event_count": 2,
-            "log_path": "20261029T124501-9f3c2a.jsonl",
+            "log_path": log_path.clone(),
             "crash_found": true,
             "diagnosis_top": "integrity.forge_missing_library"
         });
-        std::fs::write(game_dir.join("index.jsonl"), format!("{entry}\n")).expect("写索引");
+        std::fs::write(index, format!("{entry}\n")).expect("写索引");
         let events = [
             json!({"seq":0,"ts":"2026-10-29T12:45:01.123Z","t_ms":0,"kind":"stdout","payload":{"line":"hi","level":"info"}}),
             json!({"seq":1,"ts":"2026-10-29T12:45:02.123Z","t_ms":1000,"kind":"stderr","payload":{"line":"boom","level":"error"}}),
@@ -173,7 +178,7 @@ mod tests {
             .iter()
             .map(|event| format!("{event}\n"))
             .collect::<String>();
-        std::fs::write(game_dir.join("20261029T124501-9f3c2a.jsonl"), text).expect("写事件");
+        std::fs::write(session_index::event_file(root, &log_path), text).expect("写事件");
     }
 
     #[test]