feat(state.db): persist platform_message_id; restore yuanbao exact-id recall

PR #29211 dropped JSONL gateway transcripts and noted that the platform's
own `message_id` field (used by Yuanbao's recall guard to redact a
message by exact platform id) was no longer preserved — falling back to
content-match.  That fallback works for the common case but redacts the
wrong row when two messages share text (or fails to match when content
is post-processed).

Restore exact-id matching by giving state.db a column for it:

- New `platform_message_id TEXT` column on the messages table
  (SCHEMA_VERSION bump 11 → 12; column added via declarative reconciler
  on existing DBs, no version-gated migration block needed)
- Partial index `idx_messages_platform_msg_id` on
  (session_id, platform_message_id) to keep recall's point-lookup cheap
  even on large sessions
- `append_message()` and `replace_messages()` accept the new value:
  the gateway-facing `append_to_transcript` in `gateway/session.py`
  forwards either `message["platform_message_id"]` or the legacy
  `message["message_id"]` key (yuanbao's existing convention)
- `get_messages_as_conversation()` surfaces the column back on the
  message dict as `message_id` so platform code reads the same shape
  it used to read from JSONL
- Yuanbao `_patch_transcript`: restore branch A1 (exact id match)
  ahead of A2 (content match) ahead of B (system-note).  Both branches
  log which one fired so operators can tell from gateway.log whether
  recall hit the canonical path or had to fall back.

Tests:
- New low-level round-trip tests in `test_hermes_state.py` for both
  `append_message` and `replace_messages` paths
- The PR's `test_yuanbao_recall_db_only.py` was rewritten to assert
  the new contract: branch A1 (id match) works against DB-only
  transcripts, and branch A2 (content match) still recovers rows that
  were observed without a platform id (e.g. agent-processed @bot
  messages where run.py doesn't carry msg_id through)

tests/test_hermes_state.py

@@ -316,6 +316,42 @@ class TestMessageStorage:
         assert conv[0] == {"role": "user", "content": "Hello"}
         assert conv[1] == {"role": "assistant", "content": "Hi!"}
 
+    def test_platform_message_id_round_trips(self, db):
+        """Platform-side message ids (yuanbao msg_id, telegram update_id, …)
+        survive append → get_messages_as_conversation under the
+        ``message_id`` key so platform recall flows can match by exact id."""
+        db.create_session(session_id="s_pmi", source="yuanbao")
+        db.append_message(
+            "s_pmi",
+            role="user",
+            content="hi",
+            platform_message_id="abc-123",
+        )
+        db.append_message("s_pmi", role="assistant", content="hello")
+
+        conv = db.get_messages_as_conversation("s_pmi")
+        user_msg = next(m for m in conv if m["role"] == "user")
+        assistant_msg = next(m for m in conv if m["role"] == "assistant")
+        assert user_msg.get("message_id") == "abc-123"
+        # Assistant row had no platform id — must not gain one spuriously.
+        assert "message_id" not in assistant_msg
+
+    def test_replace_messages_preserves_platform_message_id(self, db):
+        """``rewrite_transcript`` (which goes through replace_messages) must
+        keep the platform_message_id round-trip working for /retry, /undo,
+        /compress and yuanbao's recall rewrite path."""
+        db.create_session(session_id="s_rep", source="yuanbao")
+        db.replace_messages(
+            "s_rep",
+            [
+                {"role": "user", "content": "x", "message_id": "ext-1"},
+                {"role": "assistant", "content": "y"},
+            ],
+        )
+        conv = db.get_messages_as_conversation("s_rep")
+        assert next(m for m in conv if m["role"] == "user").get("message_id") == "ext-1"
+        assert "message_id" not in next(m for m in conv if m["role"] == "assistant")
+
     def test_get_messages_as_conversation_includes_ancestor_chain(self, db):
         db.create_session("root", "tui")
         db.append_message("root", role="user", content="first prompt")
@@ -1462,9 +1498,10 @@ class TestSchemaInit:
         assert "schema_version" in tables
 
     def test_schema_version(self, db):
+        from hermes_state import SCHEMA_VERSION
         cursor = db._conn.execute("SELECT version FROM schema_version")
         version = cursor.fetchone()[0]
-        assert version == 11
+        assert version == SCHEMA_VERSION
 
     def test_title_column_exists(self, db):
         """Verify the title column was created in the sessions table."""
@@ -1760,8 +1797,9 @@ class TestSchemaInit:
         migrated_db = SessionDB(db_path=db_path)
 
         # Verify migration
+        from hermes_state import SCHEMA_VERSION
         cursor = migrated_db._conn.execute("SELECT version FROM schema_version")
-        assert cursor.fetchone()[0] == 11
+        assert cursor.fetchone()[0] == SCHEMA_VERSION
 
         # Verify title column exists and is NULL for existing sessions
         session = migrated_db.get_session("existing")
@@ -2952,11 +2990,12 @@ class TestFTS5ToolCallMigration:
             assert len(session_db.search_messages("LEGACYARG")) == 1, \
                 "v11 migration must backfill tool_calls JSON into FTS"
             # schema_version bumped
+            from hermes_state import SCHEMA_VERSION
             row = session_db._conn.execute(
                 "SELECT version FROM schema_version LIMIT 1"
             ).fetchone()
             version = row["version"] if hasattr(row, "keys") else row[0]
-            assert version == 11
+            assert version == SCHEMA_VERSION
         finally:
             session_db.close()