feat: let agents start new sessions via spawn_session tool

Add a project-scoped spawn_session tool so agents can dispatch new,
independent sessions (ralph loops, long-plan chaining). Spawned sessions
are constrained to a workspace/worktree of the same registered project,
appear in the session list immediately via a new session.created event,
and the capability is on by default with a Settings -> Session daemon
toggle (spawnSessions / PI_WEB_SPAWN_SESSIONS).

Note: adds a session daemon code path, so pi-web-sessiond.service must be
restarted manually for the server side to take effect.

package.json

@@ -71,6 +71,7 @@
     "lit": "^3.3.1",
     "marked": "^18.0.3",
     "node-pty": "^1.1.0",
+    "typebox": "1.1.38",
     "ws": "^8.20.1"
   },
   "devDependencies": {