feat(sessions): add tracked subsessions behind a beta flag

Add spawn_subsession / list_subsessions / read_subsession tools that let an
agent start child sessions it stays attached to: the child records its parent
in the session tree, the parent is notified (as a system-authored custom
message that wakes an idle parent and queues behind in-flight work) when the
child stops working, and the parent can inspect children's status and result.

Gated behind a beta flag, off by default, mirroring spawnSessions: enable via
PI_WEB_SUBSESSIONS, the subsessions config key, or the Settings toggle. Also
requires spawnSessions.

Also fix the release skill so the version step resyncs package-lock.json
(npm install --package-lock-only) and the commit step refuses a release where
package.json and package-lock.json versions disagree.

src/server/configRoutes.ts

@@ -112,6 +112,7 @@ function piWebConfigEnvOverrides(env: NodeJS.ProcessEnv): PiWebConfigEnvOverride
     port: isEnvSet(env["PI_WEB_PORT"]) || isEnvSet(env["PORT"]),
     allowedHosts: isEnvSet(env["PI_WEB_ALLOWED_HOSTS"]),
     spawnSessions: isEnvSet(env["PI_WEB_SPAWN_SESSIONS"]),
+    subsessions: isEnvSet(env["PI_WEB_SUBSESSIONS"]),
   };
 }