fix(uninstall): remove statusline nudge flag

[l2yujw]

Summary

The uninstall script removes Ponytail's active-mode flag and configuration, but leaves this internal marker behind:

~/.claude/.ponytail-statusline-nudged

This marker suppresses the one-time status-line setup notice. After uninstalling and later reinstalling Ponytail, a clean installation can therefore skip the notice even when no Ponytail status line is configured.

This PR removes the marker during uninstall and extends the uninstall regression test to cover it.

Root cause

hooks/ponytail-activate.js creates .ponytail-statusline-nudged, but scripts/uninstall.js did not include it in Ponytail-owned state cleanup.

Fix

Delete .ponytail-statusline-nudged alongside .ponytail-active.

The existing configuration and combined-status-line cleanup behavior is unchanged.

Verification

node --test tests/uninstall.test.js
npm test
git diff --check