docs: clarify uninstall run-order + statusLine ceiling (#278)

Follow-up to #228 (issue #226):

- README: state that scripts/uninstall.js must run *before* the host
  remove command, since the script is itself a plugin file and gets
  deleted by the removal (or run it from a separate clone).
- uninstall.js: add a ponytail: comment naming the statusLine match
  ceiling — substring match + whole-key delete removes a combined
  (e.g. caveman+ponytail) statusline wholesale; upgrade path noted.
- Add trailing newline to the file.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>