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-lock.json

@@ -31,6 +31,7 @@
         "lit": "^3.3.1",
         "marked": "^18.0.3",
         "node-pty": "^1.1.0",
+        "typebox": "1.1.38",
         "ws": "^8.20.1"
       },
       "bin": {
@@ -9050,7 +9051,6 @@
       "version": "1.1.38",
       "resolved": "https://registry.npmjs.org/typebox/-/typebox-1.1.38.tgz",
       "integrity": "sha512-pZ0aQPmMmXoUvSbeuWf/Hzsc+avNw/Zd6VeE8CFgkVGWyuHPJvqeJJDeJqLve+K70LvjYIoleGcoJHPT17cWoA==",
-      "dev": true,
       "license": "MIT"
     },
     "node_modules/typescript": {