fix(server): keep typeof getAgentDir guard for protocol-only stub

MENTOR_RUNNER_PROTOCOL_ONLY=1 swaps the Pi SDK for a deterministic stub
(buildStubSdk) that does not export getAgentDir. The previous commit
dropped the typeof guard, so the smoke test in pi-mentor-runner.spec.mjs
crashed with `getAgentDir is not a function`. Restore the guard and
update its fallback to /workspace/.pi to match the new container layout.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: FelixTJDietrich

server/application-server/src/main/resources/agent/pi-mentor-runner.mjs

@@ -225,7 +225,10 @@ async function ensureRuntime() {
             DefaultResourceLoader,
             getAgentDir,
         } = await loadSdk();
-        const agentDir = AGENT_DIR_OVERRIDE ?? getAgentDir();
+        // `typeof getAgentDir === "function"` guard accommodates MENTOR_RUNNER_PROTOCOL_ONLY=1 mode,
+        // where the stub SDK (see buildStubSdk) does not export getAgentDir. Tests then fall through
+        // to the workspace-relative default that matches production's PI_CODING_AGENT_DIR.
+        const agentDir = AGENT_DIR_OVERRIDE ?? (typeof getAgentDir === "function" ? getAgentDir() : "/workspace/.pi");
 
         // System prompt is optional in v1 — the Java caller may inject it later. If the
         // resource exists we load it once and reuse via ResourceLoader override.