fix(prd-7): M4 test coverage review — 28 new tests for comment operations and branch setup

- test/create-initial-comment.test.ts: 15 tests covering GitHub/Forgejo
  platform branches, sticky comment matching, review comment fallback,
  error handling with GITHUB_OUTPUT verification
- test/update-tracking-comment.test.ts: 11 tests covering body construction,
  branch link logic (issue vs PR), comment type routing, Forgejo URLs
- test/setup-branch-forgejo.test.ts: 2 tests verifying Forgejo uses
  repos.getBranch() while GitHub uses git.getRef() for source branch SHA
- test/COVERAGE-NOTES.md: documents all coverage status and accepted gaps
  (tag mode orchestrator, entrypoint) with rationale
- Notable finding: CLAUDE_APP_BOT_ID discrepancy flagged for M5

Progress: 67% complete (4/6 milestones) — M5 code quality next

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/setup-branch-forgejo.test.ts

@@ -0,0 +1,150 @@
+#!/usr/bin/env bun
+
+/**
+ * Tests the Forgejo API divergence in setupBranch().
+ *
+ * Forgejo uses repos.getBranch() to get source branch SHA because Forgejo's
+ * /git/refs (plural) API differs from GitHub's /git/ref (singular), causing
+ * git.getRef() to 404 on Forgejo.
+ *
+ * Approach: Call setupBranch() with mocked Octokit. The platform-specific
+ * API call records that it was called, then throws a sentinel error to stop
+ * execution before git shell commands run (avoiding real repo side effects).
+ */
+
+import {
+  describe,
+  test,
+  expect,
+  beforeEach,
+  afterEach,
+  spyOn,
+  type Mock,
+} from "bun:test";
+import { setupBranch } from "../src/github/operations/branch";
+import { _resetPlatformCache } from "../src/platform/detector";
+import { createMockContext } from "./mockContext";
+import type { Octokits } from "../src/github/api/client";
+import type { FetchDataResult } from "../src/github/data/fetcher";
+
+const mockGithubData: FetchDataResult = {
+  contextData: {
+    title: "Test issue",
+    labels: { nodes: [{ name: "bug" }] },
+  },
+  comments: [],
+  changedFiles: [],
+  changedFilesWithSHA: [],
+  reviewData: null,
+  imageUrlMap: new Map(),
+} as any;
+
+function createBranchMockOctokits(
+  overrides: Record<string, any> = {},
+): Octokits {
+  return {
+    rest: {
+      repos: {
+        get:
+          overrides.reposGet ??
+          (async () => ({ data: { default_branch: "main" } })),
+        getBranch:
+          overrides.getBranch ??
+          (async () => ({ data: { commit: { sha: "abc123" } } })),
+      },
+      git: {
+        getRef:
+          overrides.getRef ??
+          (async () => ({ data: { object: { sha: "abc123" } } })),
+      },
+    },
+    graphql: {} as any,
+  } as unknown as Octokits;
+}
+
+describe("setupBranch — Forgejo API divergence", () => {
+  let savedPlatform: string | undefined;
+  let savedServerUrl: string | undefined;
+  let consoleLogSpy: Mock<(...args: any[]) => void>;
+  let consoleErrorSpy: Mock<(...args: any[]) => void>;
+
+  beforeEach(() => {
+    _resetPlatformCache();
+    savedPlatform = process.env.PLATFORM;
+    savedServerUrl = process.env.GITHUB_SERVER_URL;
+
+    consoleLogSpy = spyOn(console, "log").mockImplementation(() => {});
+    consoleErrorSpy = spyOn(console, "error").mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    if (savedPlatform !== undefined) process.env.PLATFORM = savedPlatform;
+    else delete process.env.PLATFORM;
+    if (savedServerUrl !== undefined)
+      process.env.GITHUB_SERVER_URL = savedServerUrl;
+    else delete process.env.GITHUB_SERVER_URL;
+
+    consoleLogSpy.mockRestore();
+    consoleErrorSpy.mockRestore();
+    _resetPlatformCache();
+  });
+
+  test("on Forgejo: calls repos.getBranch() instead of git.getRef()", async () => {
+    process.env.PLATFORM = "forgejo";
+    process.env.GITHUB_SERVER_URL = "https://git.example.com";
+    _resetPlatformCache();
+
+    let getBranchCalled = false;
+    let getRefCalled = false;
+
+    const mockOctokits = createBranchMockOctokits({
+      getBranch: async () => {
+        getBranchCalled = true;
+        // Throw sentinel error to stop before git commands execute
+        throw new Error("STOP_BEFORE_GIT_COMMANDS");
+      },
+      getRef: async () => {
+        getRefCalled = true;
+        throw new Error("STOP_BEFORE_GIT_COMMANDS");
+      },
+    });
+
+    const context = createMockContext({ isPR: false, entityNumber: 1 });
+
+    await expect(
+      setupBranch(mockOctokits, mockGithubData, context),
+    ).rejects.toThrow("STOP_BEFORE_GIT_COMMANDS");
+
+    expect(getBranchCalled).toBe(true);
+    expect(getRefCalled).toBe(false);
+  });
+
+  test("on GitHub: calls git.getRef() instead of repos.getBranch()", async () => {
+    delete process.env.PLATFORM;
+    process.env.GITHUB_SERVER_URL = "https://github.com";
+    _resetPlatformCache();
+
+    let getBranchCalled = false;
+    let getRefCalled = false;
+
+    const mockOctokits = createBranchMockOctokits({
+      getBranch: async () => {
+        getBranchCalled = true;
+        throw new Error("STOP_BEFORE_GIT_COMMANDS");
+      },
+      getRef: async () => {
+        getRefCalled = true;
+        throw new Error("STOP_BEFORE_GIT_COMMANDS");
+      },
+    });
+
+    const context = createMockContext({ isPR: false, entityNumber: 1 });
+
+    await expect(
+      setupBranch(mockOctokits, mockGithubData, context),
+    ).rejects.toThrow("STOP_BEFORE_GIT_COMMANDS");
+
+    expect(getRefCalled).toBe(true);
+    expect(getBranchCalled).toBe(false);
+  });
+});