feat: habilitar WAL mode en knowledge store para concurrencia

- Ejecutar PRAGMA journal_mode=WAL al abrir la DB
- Permite múltiples lectores + single writer concurrentes
- Mejora el rendimiento del shared knowledge compartido
- Issue 0018: Shared Knowledge (fase 2a)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

shell/knowledge/store.go

@@ -58,6 +58,13 @@ func New(dir, dbPath string, logger *slog.Logger) (*FileStore, error) {
 	if err != nil {
 		return nil, fmt.Errorf("open knowledge db: %w", err)
 	}
+
+	// Enable WAL mode for better concurrency (allows multiple readers + single writer)
+	if _, err := db.Exec("PRAGMA journal_mode=WAL"); err != nil {
+		db.Close()
+		return nil, fmt.Errorf("enable WAL mode: %w", err)
+	}
+
 	if _, err := db.Exec(ftsSchema); err != nil {
 		db.Close()
 		return nil, fmt.Errorf("create knowledge fts5 table: %w", err)