feat(cli): optional notify when blocked on approval

[ZakAnun]

Summary

Adds an optional notification when the interactive CLI blocks on tool approval, so users who run ml-intern in the background get a ping without polling the terminal.

Closes #103

What changed

• New flags: --notify-on-block and --notify-method {auto,bell,desktop} (interactive mode only; headless unchanged).
• auto: tries desktop notification when available (osascript on macOS, notify-send on Linux), otherwise falls back to the terminal bell (\a).
• bell / desktop force the chosen transport; desktop failures fall back to bell where appropriate.
• Notification runs when handling approval_required and not in yolo auto-approve mode, immediately before the approval UI.
• Unit tests in tests/unit/test_main_notifications.py for the notification helpers.
• README usage line for the new flags.

Out of scope (issue listed as possibilities)

• Windows notifu (Windows still gets bell via auto when no desktop helper is wired).
• User-provided callback hook (could be a follow-up if desired).

How to test

uv sync --extra dev
uv run pytest tests/unit/test_main_notifications.py -v

[ZakAnun]

@akseljoonas Hello, should I wait for #107 to be merged and then proceed with the automatic Claude review? Because automatic review still doesn't work after resynchronizing the main upstream.