watchpr: fix watch mode hanging when a merged PR's head commit is gone

FetchState fetched the PR (merged=true) but then failed the whole state
fetch when CommitStatus 404'd for a head commit that no longer existed
(the branch was deleted after a squash/rebase merge). The merge signal was
discarded, so the watch loop treated every post-merge poll as a transient
error and never exited -- the 37-minute hang seen in production.

- Add a typed APIError carrying the HTTP status so callers can detect a 404
  without parsing error strings.
- FetchState now tolerates a 404 from CommitStatus (commit gone => no CI
  status) and returns the authoritative merged/closed PR state.
- Regression tests: FetchState survives a 404 status; the full watch loop,
  driven through a real client, detects a merge whose head commit is gone
  (both fail/hang before the fix).

internal/agent/watch.go

@@ -22,8 +22,11 @@ func FetchState(c *GiteaClient, ref PRRef, agentLogin string) (PRState, error) {
 	if err != nil {
 		return PRState{}, err
 	}
+	// A 404 here means the head commit is gone (branch deleted after a squash/
+	// rebase merge); the PR object is still authoritative, so treat CI as absent
+	// rather than discarding the merge signal and hanging the watch loop.
 	ci, err := c.CommitStatus(ref.RepoPath(), pr.Head.Sha)
-	if err != nil {
+	if err != nil && !isNotFound(err) {
 		return PRState{}, err
 	}
 	comments, err := c.ListComments(ref.RepoPath(), ref.Number)