fix: reject review as a default mode in pi-extension and config (#576)

review is a session-only mode set by /ponytail-review, never a persistent
default. The Claude/Codex mode-tracker already enforces this (#377), but two
paths did not:

- pi-extension /ponytail default review was accepted, persisting review as the
  default, so the next session started in review with a blank status-bar icon.
- getDefaultMode accepted review from PONYTAIL_DEFAULT_MODE or a stale config.

Route default validation through RUNTIME_MODES (off/lite/full/ultra) in
getDefaultMode, writeDefaultMode, and the pi-extension default command. review
stays valid as a session mode. Adds regression tests for all three paths.

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