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/client_test.go

@@ -163,6 +163,59 @@ func TestFetchState(t *testing.T) {
 	}
 }
 
+// A merged PR whose branch was deleted leaves its head commit unreachable, so
+// the commit-status endpoint 404s. FetchState must still return the (merged)
+// state rather than failing, otherwise the watch loop never sees the merge.
+func TestFetchStateToleratesMissingCommit(t *testing.T) {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/api/v1/repos/unkin/repo/pulls/7", func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"number":7,"state":"closed","merged":true,"mergeable":true,"title":"feat","html_url":"u","head":{"sha":"cafebabecafebabe"}}`)
+	})
+	mux.HandleFunc("/api/v1/repos/unkin/repo/commits/cafebabecafebabe/status", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		_, _ = io.WriteString(w, `{"message":"not found"}`)
+	})
+	mux.HandleFunc("/api/v1/repos/unkin/repo/issues/7/comments", func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `[]`)
+	})
+	srv := httptest.NewServer(mux)
+	defer srv.Close()
+
+	c := &GiteaClient{BaseURL: srv.URL, Token: "t", HTTP: srv.Client()}
+	ref := PRRef{Owner: "unkin", Repo: "repo", Number: 7}
+	st, err := FetchState(c, ref, "unkin-agent")
+	if err != nil {
+		t.Fatalf("FetchState must tolerate a 404 status for a gone commit: %v", err)
+	}
+	if !st.Merged || st.State != "closed" {
+		t.Errorf("merged/state = %t/%q, want true/closed", st.Merged, st.State)
+	}
+	if st.CIStatus != "" {
+		t.Errorf("CIStatus = %q, want empty (no status for a gone commit)", st.CIStatus)
+	}
+}
+
+// A non-404 error from the status endpoint is still fatal: only "commit gone" is
+// tolerated, not, say, an auth or server failure.
+func TestFetchStateFailsOnNon404StatusError(t *testing.T) {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/api/v1/repos/unkin/repo/pulls/7", func(w http.ResponseWriter, r *http.Request) {
+		_, _ = io.WriteString(w, `{"number":7,"state":"open","merged":false,"mergeable":true,"head":{"sha":"cafebabecafebabe"}}`)
+	})
+	mux.HandleFunc("/api/v1/repos/unkin/repo/commits/cafebabecafebabe/status", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		_, _ = io.WriteString(w, `{"message":"boom"}`)
+	})
+	srv := httptest.NewServer(mux)
+	defer srv.Close()
+
+	c := &GiteaClient{BaseURL: srv.URL, Token: "t", HTTP: srv.Client()}
+	ref := PRRef{Owner: "unkin", Repo: "repo", Number: 7}
+	if _, err := FetchState(c, ref, "unkin-agent"); err == nil {
+		t.Fatal("FetchState should surface a 500 from the status endpoint")
+	}
+}
+
 func TestGiteaAPIError(t *testing.T) {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/api/v1/repos/unkin/repo/pulls", func(w http.ResponseWriter, r *http.Request) {