feat(projects): notify enabled plugins on project deletion (project_deleted hook)

[Jehu]

Motivation

Deleting a project can leave external, per-project resources behind (for example per-project memory banks used by memory plugins). Plugins currently have no reliable signal that a project was removed, which leads to orphaned external state.

What it does

delete_project() now notifies all enabled plugins after the project directory is removed and the project is deactivated in chats:

• New project_deleted(project_name: str, **kwargs) hook, implemented by plugins in their hooks.py.
• Notification goes to every enabled plugin via the existing call_plugin_hook mechanism; plugins without the hook are skipped silently.

Implementation notes

• Fully additive: one call in delete_project() plus a small private helper _notify_project_deleted() in helpers/projects.py.
• No new module-level imports (deferred import avoids import cycles).
• Fail-safe by design: plugin enumeration and every hook call are guarded, so hook errors can never break project deletion.

Tests

New tests/test_project_deleted_hook.py covers:

• enabled plugin hook receives the correct project name
• raising hook does not break deletion
• disabled plugins are not notified
• plugins without hooks.py are harmless

All 4 new tests pass, plus the existing tests/test_projects.py suite (20 passed) on Python 3.12.

Out of scope

Project renaming (and a corresponding project_renamed hook) is intentionally not included; renaming project directories is not currently supported by the UI and would be a separate, larger change.