Add agentpr pr edit subcommand

Update a PR's title and/or body as the agent user, sending only the
fields supplied.

internal/agent/client_test.go

@@ -98,6 +98,80 @@ func TestCreatePRRequestBody(t *testing.T) {
 	}
 }
 
+// An edit must send only the fields it was given: Gitea overwrites whatever
+// key it receives, so an omitted --title arriving as "" would blank the title.
+func TestEditPRSendsOnlySuppliedFields(t *testing.T) {
+	title, body, empty := "new title", "new body", ""
+	tests := []struct {
+		name string
+		opts EditPROptions
+		want map[string]any
+	}{
+		{"body only", EditPROptions{Body: &body}, map[string]any{"body": "new body"}},
+		{"title only", EditPROptions{Title: &title}, map[string]any{"title": "new title"}},
+		{"both", EditPROptions{Title: &title, Body: &body}, map[string]any{"title": "new title", "body": "new body"}},
+		{"explicit empty body is sent", EditPROptions{Body: &empty}, map[string]any{"body": ""}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var gotBody map[string]any
+			var gotMethod, gotPath string
+			mux := http.NewServeMux()
+			mux.HandleFunc("/api/v1/repos/unkin/repo/pulls/7", func(w http.ResponseWriter, r *http.Request) {
+				gotMethod, gotPath = r.Method, r.URL.Path
+				_ = json.NewDecoder(r.Body).Decode(&gotBody)
+				_, _ = io.WriteString(w, `{"number":7,"title":"new title","html_url":"https://git.unkin.net/unkin/repo/pulls/7"}`)
+			})
+			srv := httptest.NewServer(mux)
+			defer srv.Close()
+
+			c := &GiteaClient{BaseURL: srv.URL, Token: "gitea-abc", HTTP: srv.Client()}
+			pr, err := c.EditPR("unkin/repo", 7, tt.opts)
+			if err != nil {
+				t.Fatalf("EditPR: %v", err)
+			}
+			if gotMethod != http.MethodPatch {
+				t.Errorf("method = %s, want PATCH", gotMethod)
+			}
+			if gotPath != "/api/v1/repos/unkin/repo/pulls/7" {
+				t.Errorf("path = %q", gotPath)
+			}
+			if len(gotBody) != len(tt.want) {
+				t.Errorf("payload = %v, want exactly the supplied fields %v", gotBody, tt.want)
+			}
+			for k, v := range tt.want {
+				if gotBody[k] != v {
+					t.Errorf("payload[%q] = %v, want %v", k, gotBody[k], v)
+				}
+			}
+			if pr.Number != 7 || pr.HTMLURL == "" {
+				t.Errorf("parsed PR = %+v", pr)
+			}
+		})
+	}
+}
+
+// A non-2xx must surface the API's own message rather than a bare status.
+func TestEditPRAPIError(t *testing.T) {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/api/v1/repos/unkin/repo/pulls/7", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusUnprocessableEntity)
+		_, _ = io.WriteString(w, `{"message":"title cannot be empty"}`)
+	})
+	srv := httptest.NewServer(mux)
+	defer srv.Close()
+
+	title := ""
+	c := &GiteaClient{BaseURL: srv.URL, Token: "t", HTTP: srv.Client()}
+	_, err := c.EditPR("unkin/repo", 7, EditPROptions{Title: &title})
+	if err == nil {
+		t.Fatal("expected error on 422")
+	}
+	if !strings.Contains(err.Error(), "title cannot be empty") {
+		t.Errorf("error %q should carry the API message", err)
+	}
+}
+
 func TestCreateComment(t *testing.T) {
 	var gotBody map[string]string
 	mux := http.NewServeMux()

internal/agent/gitea.go

@@ -214,6 +214,22 @@ func (c *GiteaClient) CreatePR(repoPath string, opts CreatePROptions) (PullReque
 	return pr, err
 }
 
+// EditPROptions are the fields an edit may change. Pointers so an unset field
+// is omitted from the payload entirely: Gitea overwrites whatever it is sent,
+// so a nil Title leaves the title alone while a pointer to "" clears it.
+type EditPROptions struct {
+	Title *string `json:"title,omitempty"`
+	Body  *string `json:"body,omitempty"`
+}
+
+// EditPR updates a pull request's title and/or body
+// (PATCH /api/v1/repos/{owner}/{repo}/pulls/{index}).
+func (c *GiteaClient) EditPR(repoPath string, number int, opts EditPROptions) (PullRequest, error) {
+	var pr PullRequest
+	err := c.do(http.MethodPatch, fmt.Sprintf("/api/v1/repos/%s/pulls/%d", repoPath, number), opts, &pr)
+	return pr, err
+}
+
 // GetPR fetches a single pull request.
 func (c *GiteaClient) GetPR(repoPath string, number int) (PullRequest, error) {
 	var pr PullRequest