Two fixes for Discord exec approval:
1. Register /approve and /deny as native Discord slash commands so they
appear in Discord's command picker (autocomplete). Previously they
were only handled as text commands, so users saw 'no commands found'
when typing /approve.
2. Wire up the existing ExecApprovalView button UI (was dead code):
- ExecApprovalView now calls resolve_gateway_approval() to actually
unblock the waiting agent thread when a button is clicked
- Gateway's _approval_notify_sync() detects adapters with
send_exec_approval() and routes through the button UI
- Added 'Allow Session' button for parity with /approve session
- send_exec_approval() now accepts session_key and metadata for
thread support
- Graceful fallback to text-based /approve prompt if button send fails
Also updates test mocks to include grey/secondary ButtonStyle and
purple Color (used by new button styles).
