CHANGES(docs): Correct lifecycle hook semantics

why: Keep the changelog aligned with the lifecycle behavior shipped on this branch so readers do not infer broader hook semantics than the code implements.

what:
- Document on_project_start as running only for new session creation
- Document on_project_exit as running when the last client detaches
- Match the guarded client-detached hook behavior in WorkspaceBuilder

Author: tony

CHANGES

@@ -73,7 +73,7 @@ Workspace configs now support four lifecycle hooks inspired by tmuxinator:
 
 - `on_project_start` — runs before session build (new session creation only)
 - `on_project_restart` — runs when reattaching to an existing session (confirmed attach only)
-- `on_project_exit` — runs on client detach (via tmux `client-detached` hook; fires on every detach event)
+- `on_project_exit` — runs when the last client detaches (via tmux `client-detached` hook)
 - `on_project_stop` — runs before `tmuxp stop` kills the session
 
 ### Config templating (#1025)