fix(sessions): warn once per state.db when 'source' column is missing

Observed: on a multi-profile install where one profile's state.db predates
the agent's `source` column, errors.log gained one identical line every
~15 s (276 lines in ~70 min):

  api.models WARNING agent session listing skipped: state.db at
  /data/hermes/profiles/planner/state.db has no 'source' column (older
  hermes-agent?). Agent sessions unavailable. Upgrade hermes-agent to
  fix this.

Mechanism: api/agent_sessions.py read_importable_agent_session_rows()
runs `PRAGMA table_info(sessions)` and warns unconditionally when the
column is absent (api/agent_sessions.py:564 before this change).
api/models.py get_cli_sessions(all_profiles=True) calls it for every
profile on every sidebar poll, behind only the 5 s
_CLI_SESSIONS_CACHE_TTL_SECONDS cache, so the same DB re-emits the same
warning on every cache miss. The condition is a property of the DB
file, not of the poll.

Fix: a module-level `_SOURCE_COLUMN_WARNED_DB_PATHS: set[str]` keyed by
the resolved db_path. The warning fires the first time a given path is
seen in the process and is suppressed after that; a different path
warns on its own. Message text is unchanged. Process-lifetime only: a
restart warns again, which is intended (the line remains the operator's
cue that the agent still needs upgrading). Plain set mutation under the
GIL is sufficient; a duplicate line from two racing first calls is
harmless.

Tests (tests/test_issue634.py, real pre-`source` sqlite DBs under
tmp_path, asserting on captured log records):
- same db_path called twice -> exactly one WARNING (failed before:
  `assert 2 == 1`)
- two db_paths, then the first again -> exactly two WARNINGs, one per
  path (failed before: `assert 3 == 2`)
The seven existing #634 source-shape tests still pass; neighbours
test_issue3762/3930/5455/1494 pass.

Deliberately not changed: the exception-path warning in
get_cli_sessions() (api/models.py), the read-only-open fallback warning,
and the 5 s cache TTL. The operator-side root cause (the agent
reconciler creating a pre-`source` state.db) is a separate upstream fix;
this only stops the repetition.

api/agent_sessions.py

@@ -6,6 +6,17 @@ from pathlib import Path
 
 logger = logging.getLogger(__name__)
 
+# state.db paths that already produced the "no 'source' column" warning below.
+# ``get_cli_sessions(all_profiles=True)`` re-reads every profile DB on every
+# sidebar poll (behind a 5 s cache), so a single pre-``source`` profile DB would
+# otherwise re-emit the identical WARNING line every ~15 s for the life of the
+# process. The condition is a property of the DB file, not of the poll, so it
+# is reported once per path. Process-lifetime only: a restart warns again,
+# which is the desired behaviour (the log line is the operator's cue that the
+# agent still needs upgrading). Plain ``set`` mutation under the GIL is
+# sufficient here; a duplicate line from two racing first calls is harmless.
+_SOURCE_COLUMN_WARNED_DB_PATHS: set[str] = set()
+
 
 def open_state_db_readonly(db_path: Path, log: logging.Logger | None = None) -> sqlite3.Connection:
     """Open the live agent ``state.db`` read-only for a pure-read projection.
@@ -562,12 +573,15 @@ def read_importable_agent_session_rows(
         cur.execute("PRAGMA table_info(messages)")
         message_cols = {row[1] for row in cur.fetchall()}
         if 'source' not in session_cols:
-            log.warning(
-                "agent session listing skipped: state.db at %s has no 'source' column "
-                "(older hermes-agent?). Agent sessions unavailable. "
-                "Upgrade hermes-agent to fix this.",
-                db_path,
-            )
+            warned_key = str(db_path.resolve())
+            if warned_key not in _SOURCE_COLUMN_WARNED_DB_PATHS:
+                _SOURCE_COLUMN_WARNED_DB_PATHS.add(warned_key)
+                log.warning(
+                    "agent session listing skipped: state.db at %s has no 'source' column "
+                    "(older hermes-agent?). Agent sessions unavailable. "
+                    "Upgrade hermes-agent to fix this.",
+                    db_path,
+                )
             return []
 
         parent_expr = _optional_col('parent_session_id', session_cols)