fix: wire plan-gate activation and add express.json to steer/plan-gate routes

- Wire planGateRuntime.activate() into session creation path when planGate is true
  (Bug 1: sessions map stayed empty, plan/status always returned none)
- Add express.json({ limit: '1mb' }) to session-steer and plan-gate POST routes
  (Bug 2: req.body was undefined, POST with JSON body returned 400)
- Pass planGateRuntime dependency to createOpenChamberSessionService
- Add route-level and integration tests for both fixes

packages/web/server/lib/openchamber-sessions/routes.js

@@ -416,6 +416,7 @@ export const createOpenChamberSessionService = (dependencies) => {
     emitSessionCreatedEvent,
     createSessionGoal: createSessionGoalOverride,
     sessionKnowledgeRuntime = null,
+    planGateRuntime = null,
   } = dependencies;
 
   // Last user message of an existing session, as a selection to reuse. Returns
@@ -758,6 +759,10 @@ export const createOpenChamberSessionService = (dependencies) => {
       ...(cardID ? { cardID } : {}),
     });
 
+    if (planGate && planGateRuntime && typeof planGateRuntime.activate === 'function') {
+      planGateRuntime.activate(sessionID, sessionDirectory, cardID || '');
+    }
+
     let dispatch = { model, agent, variant, promptDispatched: false, dispatchedAsCommand: false };
     if (prompt) {
       dispatch = await dispatchPrompt({