feat(history): 消息搜索/编辑/删除 + 会话置顶(D2/D3/D5)

- history/message_ops.go(新): Search(会话内, q/after互斥, LIKE参数化+%/_转义+大小写不敏感, 严格限定会话doc) / UpdateMessage(返回更新后Message) / DeleteMessage / messageByID / escapeLike
- history/session_pin.go(新): SetPinned; 迁移0009 chat_sessions加pinned INTEGER DEFAULT 0; store.go SessionMeta.Pinned + createSessionsTable加列
- session_preview.go: 查询加s.pinned + ORDER BY pinned DESC,(type=main) DESC,created_at ASC
- gateway history_write.go(新,保持history.go≤500): handlePatchSession(pinned+title) / handlePatchMessage / handleDeleteMessage; server.go加PATCH/DELETE /api/v1/messages/{id} + /sessions/{id}置顶
- history.go: handleListMessages读q分流Search; 契约PATCH title/置顶
- D2会话内搜索(不做跨会话,隔离会话不外泄红线); D3不做重发端点(客户端编辑后再发); D5用pinned bool
- 测试(message_ops/history_messages/migrate_pinned), build/vet/test绿+3平台交叉编译; Docs/api-client v0.30
- 未做: 编辑/删除WS实时广播(message_edited/deleted), 下批

internal/db/migrations/0009_chat_session_pinned.down.sql

@@ -0,0 +1,6 @@
+-- 0009_chat_session_pinned down: drop the pinned ordering column added by the up
+-- migration. SQLite DROP COLUMN (3.35+) is already relied on by 0006/0007, and the
+-- column is not referenced by any index, so a plain drop is safe and lossless (the
+-- pin is order metadata, not data the session needs to remain valid).
+
+ALTER TABLE chat_sessions DROP COLUMN pinned;

internal/db/migrations/0009_chat_session_pinned.up.sql

@@ -0,0 +1,12 @@
+-- 0009_chat_session_pinned: pin a persistent chat session to the top of the session
+-- list (DevDocs/design/16 §2.2, Android 协同批次 ③a / D5). `pinned` is display/order
+-- metadata only: it never affects identity or permissions (project red line), and any
+-- session — the fixed main session or a sub session — may be pinned.
+--
+-- The 0003 chat_sessions table already exists at this point, so ALTER ADD COLUMN is
+-- the correct, non-destructive step. The canonical schema (including this column) is
+-- also re-issued idempotently by internal/runtime/history.NewStore so a fresh
+-- in-memory store used by tests stays self-contained; the migration is the
+-- authoritative path for an existing database.
+
+ALTER TABLE chat_sessions ADD COLUMN pinned INTEGER NOT NULL DEFAULT 0;