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.
