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/sessions/piSessionManagerGateway.ts

@@ -63,9 +63,9 @@ class SettingsAwarePiSessionManagerGateway implements PiSessionManagerGateway {
     return filterSessionsForCwd(await listSessionsInDir(resolution.sessionDir), cwd);
   }
 
-  create(cwd: string): PiSessionManager {
+  create(cwd: string, options?: { parentSession?: string }): PiSessionManager {
     const resolution = this.resolver.resolve(cwd);
-    return SessionManager.create(cwd, resolution.sessionDir);
+    return SessionManager.create(cwd, resolution.sessionDir, options?.parentSession === undefined ? undefined : { parentSession: options.parentSession });
   }
 
   listAll(): Promise<PiSessionListEntry[]> {