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/shared/apiTypes.ts

@@ -61,6 +61,12 @@ export interface PiWebConfigValues {
   maxUploadBytes?: number;
   /** When true, LLMs can start new sessions via the spawn_session tool. */
   spawnSessions?: boolean;
+  /**
+   * Beta: when true, LLMs can start tracked child sessions via the
+   * spawn_subsession / list_subsessions / read_subsession tools. Off by default
+   * while the capability stabilizes. Requires spawnSessions to be enabled.
+   */
+  subsessions?: boolean;
 }
 
 export type PiWebPluginScope = "bundled" | "local" | "user" | "project";
@@ -83,6 +89,7 @@ export interface PiWebConfigEnvOverrides {
   port: boolean;
   allowedHosts: boolean;
   spawnSessions: boolean;
+  subsessions: boolean;
 }
 
 export interface PiWebConfigResponse {