feat: add issue_list_comments tool (closes #32)

Lists all comments on an issue or PR via GET /api/v1/repos/{owner}/{repo}/issues/{index}/comments.
Read-only, allowlist-gated. Extended IssueComment struct with user/timestamps populated by the list endpoint.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/gitea/issues_test.go

@@ -101,3 +101,51 @@ func TestCreateIssueComment(t *testing.T) {
 	assert.Equal(t, "hello", comment.Body)
 	assert.Equal(t, "http://example.com/issues/42#comment-7", comment.HTMLURL)
 }
+
+func TestListIssueComments(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, http.MethodGet, r.Method)
+		assert.Equal(t, "/api/v1/repos/o/r/issues/42/comments", r.URL.Path)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`[
+			{"id":1,"body":"first","html_url":"http://example.com/issues/42#comment-1","user":{"login":"alice"},"created_at":"2026-05-01T00:00:00Z","updated_at":"2026-05-01T00:00:00Z"},
+			{"id":2,"body":"second","html_url":"http://example.com/issues/42#comment-2","user":{"login":"bob"},"created_at":"2026-05-02T00:00:00Z","updated_at":"2026-05-02T00:00:00Z"}
+		]`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	comments, err := c.ListIssueComments(context.Background(), "o", "r", 42)
+	require.NoError(t, err)
+	require.Len(t, comments, 2)
+	assert.Equal(t, int64(1), comments[0].ID)
+	assert.Equal(t, "first", comments[0].Body)
+	assert.Equal(t, "alice", comments[0].User.Login)
+	assert.Equal(t, "bob", comments[1].User.Login)
+}
+
+func TestListIssueComments_Empty(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`[]`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	comments, err := c.ListIssueComments(context.Background(), "o", "r", 42)
+	require.NoError(t, err)
+	assert.Empty(t, comments)
+}
+
+func TestListIssueComments_NotFound(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		_, _ = w.Write([]byte(`{"message":"issue not found"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	_, err := c.ListIssueComments(context.Background(), "o", "r", 999)
+	require.Error(t, err)
+	assert.ErrorIs(t, err, gitea.ErrNotFound)
+}