fix: bound SQLite WAL growth and prevent worker hangs (#49)

The worker's SQLite WAL could grow unbounded (observed 19MB) and wedge the
DB, hanging Claude Code on every prompt. No checkpoint ever truncated the
WAL (only PASSIVE auto-checkpoint, which cannot reclaim the file), the
connection-scoped pragmas were set via a single Exec so only one pooled
connection received them (e.g. busy_timeout=0 on the rest), and the
maintenance service that would optimize/checkpoint was never wired up.

- Register a sqlite3 ConnectHook driver so all pragmas (busy_timeout,
  journal_mode, synchronous, cache_size, foreign_keys, journal_size_limit)
  apply to every pooled connection; enable safe connection recycling.
- Add Store.Checkpoint (TRUNCATE), checkpoint-on-Close, and a periodic
  size-gated checkpoint loop with configurable interval/threshold.
- Wire up the previously-dead maintenance service; make trigger_maintenance
  actually run DB maintenance instead of only recalculating scores.
- Harden the user-prompt hook to honor its deadline and fail open so a
  slow worker can never stall a prompt.
- Add regression tests for WAL truncation, checkpoint-on-close, and
  per-connection pragmas.

internal/mcp/server.go

@@ -534,7 +534,7 @@ func (s *Server) handleToolsList(req *Request) *Response {
 		},
 		{
 			Name:        "trigger_maintenance",
-			Description: "Trigger an immediate maintenance run (cleanup old observations, optimize database).",
+			Description: "Trigger an immediate database maintenance run: optimize/checkpoint the database, clean up old prompts, apply any configured observation retention/stale cleanup, and recalculate importance scores.",
 			InputSchema: map[string]any{
 				"type":       "object",
 				"properties": map[string]any{},
@@ -917,7 +917,7 @@ func (s *Server) callTool(ctx context.Context, name string, args json.RawMessage
 			"project": s.project,
 		})
 	case "trigger_maintenance":
-		return s.proxyPostRaw(ctx, "/api/scoring/recalculate", nil)
+		return s.proxyPostRaw(ctx, "/api/maintenance/run", nil)
 	case "analyze_observation_importance":
 		return s.handleAnalyzeImportanceProxy(ctx, args)
 	case "analyze_search_patterns":