This adds five new webhook event types alongside the existing `push` and `repository:renamed` events: - `pull_request:created` - `pull_request:resubmitted` - `pull_request:merged` - `pull_request:closed` - `pull_request:reopened` Each event carries a payload with an `action` field, pull request metadata (number, title, state, target branch, source branch/repo/sha), the repository object, and the sender. The patch itself is not embedded; payloads include a `patch_url` pointing at the existing raw round-patch route instead. Merge, close, and reopen reuse the existing `NewPullState` notifier call sites. `Resubmission` previously emitted no notifier event at all, so this adds a `ResubmitPull` method to the `Notifier` interface, fired from both the regular and stacked resubmit paths (stacked resubmits also now fire `NewPull` for pulls newly added to a stack). Stack-pull abandonment intentionally emits no event. The webhook notifier now takes the appview base URL so pull request `html_url`/`patch_url` point at the appview, where pull routes live (`repository.html_url` keeps pointing at the knot, unchanged). Refs #97, #94.