fix: session_search nil store panic — graceful error instead of crash

The sessionSearchTool is created with a nil store when builtinTools
is called without --session (odek run, subagent). Any invocation
of the session_search tool panics with nil pointer dereference.

Fix: add an early nil-check in Call() that returns a clear error
message: "session store is not available (use --session to enable
session persistence)" instead of panicking and being caught by the
defer/recover.

This was discovered when the agent tried to use session_search to
look up past audit findings.

Author: molty3000

cmd/odek/session_search_tool.go

@@ -87,6 +87,11 @@ func (t *sessionSearchTool) Call(argsJSON string) (result string, err error) {
 		}
 	}()
 
+	// Guard: store must be initialized (nil when --session is not used).
+	if t.store == nil {
+		return jsonError("session store is not available (use --session to enable session persistence)")
+	}
+
 	var args sessionSearchArgs
 	if err := json.Unmarshal([]byte(argsJSON), &args); err != nil {
 		return jsonError("invalid arguments: " + err.Error())