feat(tbd_ship): idempotent re-invoke — resume existing branch/PR (#48)

A second tbd_ship for the same change no longer errors on "branch exists":
CreateBranch conflict is tolerated, and if a PR is already open for the head,
CreatePullRequest's conflict/validation error resolves it via ListPullRequests
(matching head.ref). Identical file content on the branch skips the write, so a
resume produces no redundant empty-diff commit. Then the same CI gate runs and
merges if now green — so "poll or re-invoke" (the #40 UX) actually works.

Test: TestTBDShip_Resume_ExistingBranchAndPR (branch+PR exist, content
unchanged → no write, merges when green). First-call paths unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/tools/tbd_ship_test.go

@@ -150,6 +150,60 @@ func TestTBDShip_MergeConflict_FailsClosed(t *testing.T) {
 	assert.False(t, deleted.Load(), "branch is NOT deleted on a failed merge")
 }
 
+// Re-invoking with the same change must resume the existing branch/PR (not
+// error on "branch exists"), skip the redundant write when content is
+// unchanged, and merge once CI is green (#48).
+func TestTBDShip_Resume_ExistingBranchAndPR(t *testing.T) {
+	var merged, deleted, wrote atomic.Bool
+	branch := "tbd/resume-me"
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		p := r.URL.Path
+		w.Header().Set("Content-Type", "application/json")
+		switch {
+		case r.Method == http.MethodGet && strings.Contains(p, "/branch_protections/"):
+			w.WriteHeader(http.StatusNotFound)
+			_, _ = w.Write([]byte(`{"message":"not found"}`))
+		case r.Method == http.MethodPost && strings.HasSuffix(p, "/branches"):
+			w.WriteHeader(http.StatusConflict) // branch already exists
+			_, _ = w.Write([]byte(`{"message":"branch already exists"}`))
+		case r.Method == http.MethodGet && strings.Contains(p, "/contents/"):
+			// existing file with identical content ("hi") → write should be skipped
+			w.WriteHeader(http.StatusOK)
+			_, _ = w.Write([]byte(`{"path":"docs/x.md","sha":"s","content":"aGk=","encoding":"base64"}`))
+		case (r.Method == http.MethodPost || r.Method == http.MethodPut) && strings.Contains(p, "/contents/"):
+			wrote.Store(true)
+			w.WriteHeader(http.StatusCreated)
+			_, _ = w.Write([]byte(`{"content":{"path":"x","sha":"s2"},"commit":{"sha":"c"}}`))
+		case r.Method == http.MethodPost && strings.HasSuffix(p, "/pulls"):
+			w.WriteHeader(http.StatusConflict) // PR already exists for this head
+			_, _ = w.Write([]byte(`{"message":"pull request already exists"}`))
+		case r.Method == http.MethodGet && strings.HasSuffix(p, "/pulls"):
+			_, _ = w.Write([]byte(`[{"number":7,"html_url":"http://x/pulls/7","state":"open","head":{"ref":"` + branch + `","sha":"abc"},"base":{"ref":"main"}}]`))
+		case r.Method == http.MethodGet && strings.Contains(p, "/actions/runs"):
+			_, _ = w.Write([]byte(`{"total_count":1,"workflow_runs":[{"id":1,"status":"completed","conclusion":"success","head_sha":"abc"}]}`))
+		case r.Method == http.MethodPost && strings.HasSuffix(p, "/pulls/7/merge"):
+			merged.Store(true)
+			w.WriteHeader(http.StatusOK)
+			_, _ = w.Write([]byte(`{}`))
+		case r.Method == http.MethodDelete && strings.Contains(p, "/branches/"):
+			deleted.Store(true)
+			w.WriteHeader(http.StatusOK)
+			_, _ = w.Write([]byte(`{}`))
+		default:
+			t.Errorf("unexpected request: %s %s", r.Method, p)
+			w.WriteHeader(http.StatusNotFound)
+		}
+	}))
+	defer srv.Close()
+
+	res := callShip(t, srv.URL, `{"owner":"mathias","repo":"myrepo","path":"docs/x.md","content":"hi","message":"add x","branch":"`+branch+`"}`)
+
+	assert.Equal(t, true, res["merged"], "resume must merge when CI is green")
+	assert.Equal(t, float64(7), res["pr_number"], "must reuse the existing PR #7")
+	assert.False(t, wrote.Load(), "identical content must not trigger a redundant write")
+	assert.True(t, merged.Load())
+}
+
 func TestTBDShip_AllowlistRejects(t *testing.T) {
 	tool := tools.NewTBDShip(gitea.NewClient("http://unused", ""), allowlist.New([]string{"mathias"}))
 	_, err := tool.Call(context.Background(), json.RawMessage(`{"owner":"evil","repo":"r","path":"p","content":"c","message":"m"}`))