feat(state): auto-prune old sessions + VACUUM state.db at startup (#13861)

* feat(state): auto-prune old sessions + VACUUM state.db at startup

state.db accumulates every session, message, and FTS5 index entry forever.
A heavy user (gateway + cron) reported 384MB with 982 sessions / 68K messages
causing slowdown; manual 'hermes sessions prune --older-than 7' + VACUUM
brought it to 43MB. The prune command and VACUUM are not wired to run
automatically anywhere — sessions grew unbounded until users noticed.

Changes:
- hermes_state.py: new state_meta key/value table, vacuum() method, and
  maybe_auto_prune_and_vacuum() — idempotent via last-run timestamp in
  state_meta so it only actually executes once per min_interval_hours
  across all Hermes processes for a given HERMES_HOME. Never raises.
- hermes_cli/config.py: new 'sessions:' block in DEFAULT_CONFIG
  (auto_prune=True, retention_days=90, vacuum_after_prune=True,
  min_interval_hours=24). Added to _KNOWN_ROOT_KEYS.
- cli.py: call maintenance once at HermesCLI init (shared helper
  _run_state_db_auto_maintenance reads config and delegates to DB).
- gateway/run.py: call maintenance once at GatewayRunner init.
- Docs: user-guide/sessions.md rewrites 'Automatic Cleanup' section.

Why VACUUM matters: SQLite does NOT shrink the file on DELETE — freed
pages get reused on next INSERT. Without VACUUM, a delete-heavy DB stays
bloated forever. VACUUM only runs when the prune actually removed rows,
so tight DBs don't pay the I/O cost.

Tests: 10 new tests in tests/test_hermes_state.py covering state_meta,
vacuum, idempotency, interval skipping, VACUUM-only-when-needed,
corrupt-marker recovery. All 246 existing state/config/gateway tests
still pass.

Verified E2E with real imports + isolated HERMES_HOME: DEFAULT_CONFIG
exposes the new block, load_config() returns it for fresh installs,
first call prunes+vacuums, second call within min_interval_hours skips,
and the state_meta marker persists across connection close/reopen.

* sessions.auto_prune defaults to false (opt-in)

Session history powers session_search recall across past conversations,
so silently pruning on startup could surprise users. Ship the machinery
disabled and let users opt in when they notice state.db is hurting
performance.

- DEFAULT_CONFIG.sessions.auto_prune: True → False
- Call-site fallbacks in cli.py and gateway/run.py match the new default
  (so unmigrated configs still see off)
- Docs: flip 'Enable in config.yaml' framing + tip explains the tradeoff

cli.py

@@ -914,6 +914,32 @@ def _cleanup_worktree(info: Dict[str, str] = None) -> None:
     print(f"\033[32m✓ Worktree cleaned up: {wt_path}\033[0m")
 
 
+def _run_state_db_auto_maintenance(session_db) -> None:
+    """Call ``SessionDB.maybe_auto_prune_and_vacuum`` using current config.
+
+    Reads the ``sessions:`` section from config.yaml via
+    :func:`hermes_cli.config.load_config` (the authoritative loader that
+    deep-merges DEFAULT_CONFIG, so unmigrated configs still get default
+    values). Honours ``auto_prune`` / ``retention_days`` /
+    ``vacuum_after_prune`` / ``min_interval_hours``, and delegates to the
+    DB. Never raises — maintenance must never block interactive startup.
+    """
+    if session_db is None:
+        return
+    try:
+        from hermes_cli.config import load_config as _load_full_config
+        cfg = (_load_full_config().get("sessions") or {})
+        if not cfg.get("auto_prune", False):
+            return
+        session_db.maybe_auto_prune_and_vacuum(
+            retention_days=int(cfg.get("retention_days", 90)),
+            min_interval_hours=int(cfg.get("min_interval_hours", 24)),
+            vacuum=bool(cfg.get("vacuum_after_prune", True)),
+        )
+    except Exception as exc:
+        logger.debug("state.db auto-maintenance skipped: %s", exc)
+
+
 def _prune_stale_worktrees(repo_root: str, max_age_hours: int = 24) -> None:
     """Remove stale worktrees and orphaned branches on startup.
 
@@ -1961,7 +1987,13 @@ class HermesCLI:
             self._session_db = SessionDB()
         except Exception as e:
             logger.warning("Failed to initialize SessionDB — session will NOT be indexed for search: %s", e)
-        
+
+        # Opportunistic state.db maintenance — runs at most once per
+        # min_interval_hours, tracked via state_meta in state.db itself so
+        # it's shared across all Hermes processes for this HERMES_HOME.
+        # Never blocks startup on failure.
+        _run_state_db_auto_maintenance(self._session_db)
+
         # Deferred title: stored in memory until the session is created in the DB
         self._pending_title: Optional[str] = None