feat(prd-1): implement M6 - MCP server platform routing

- Add platform detection to prepareMcpConfig() via detectPlatform()
- Skip GitHub public MCP server (ghcr.io/github/github-mcp-server) on Forgejo
- Skip checkActionsReadPermission on Forgejo (workflow token has full access)
- Reuse existing github-actions-server.ts for both platforms (no duplication)
- Add 6 Forgejo-specific tests to install-mcp-server.test.ts
- Updated PRD checkboxes for M6

Progress: 67% complete (6/9 milestones) - M7 now unblocked

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

test/install-mcp-server.test.ts

@@ -313,4 +313,178 @@ describe("prepareMcpConfig", () => {
     const parsed = JSON.parse(result);
     expect(parsed.mcpServers.github_ci).not.toBeDefined();
   });
+
+  describe("Forgejo platform routing", () => {
+    let savedPlatform: string | undefined;
+    let savedForgejo: string | undefined;
+    let savedServerUrl: string | undefined;
+
+    beforeEach(() => {
+      savedPlatform = process.env.PLATFORM;
+      savedForgejo = process.env.FORGEJO;
+      savedServerUrl = process.env.GITHUB_SERVER_URL;
+      // Set platform to forgejo for all tests in this block
+      process.env.PLATFORM = "forgejo";
+    });
+
+    afterEach(() => {
+      if (savedPlatform !== undefined) {
+        process.env.PLATFORM = savedPlatform;
+      } else {
+        delete process.env.PLATFORM;
+      }
+      if (savedForgejo !== undefined) {
+        process.env.FORGEJO = savedForgejo;
+      } else {
+        delete process.env.FORGEJO;
+      }
+      if (savedServerUrl !== undefined) {
+        process.env.GITHUB_SERVER_URL = savedServerUrl;
+      } else {
+        delete process.env.GITHUB_SERVER_URL;
+      }
+    });
+
+    test("should skip GitHub public MCP server on Forgejo", async () => {
+      const result = await prepareMcpConfig({
+        githubToken: "test-token",
+        owner: "test-owner",
+        repo: "test-repo",
+        branch: "test-branch",
+        baseBranch: "main",
+        allowedTools: [
+          "mcp__github__create_issue",
+          "mcp__github__create_pr",
+        ],
+        mode: "tag",
+        context: mockContext,
+      });
+
+      const parsed = JSON.parse(result);
+      // The Docker-based GitHub MCP server should NOT be included on Forgejo
+      expect(parsed.mcpServers.github).not.toBeDefined();
+    });
+
+    test("should include CI server on Forgejo without permission check", async () => {
+      process.env.DEFAULT_WORKFLOW_TOKEN = "forgejo-workflow-token";
+      // Simulate a 403 from the permission check — on Forgejo this should be
+      // skipped entirely, so the CI server is still included.
+      fetchSpy.mockResolvedValue(
+        new Response(
+          JSON.stringify({
+            message: "Resource not accessible by integration",
+          }),
+          { status: 403 },
+        ),
+      );
+
+      const result = await prepareMcpConfig({
+        githubToken: "test-token",
+        owner: "test-owner",
+        repo: "test-repo",
+        branch: "test-branch",
+        baseBranch: "main",
+        allowedTools: [],
+        mode: "tag",
+        context: mockPRContext,
+      });
+
+      const parsed = JSON.parse(result);
+      // CI server should be included on Forgejo even though the permission
+      // check would fail — the check is skipped entirely.
+      expect(parsed.mcpServers.github_ci).toBeDefined();
+      expect(parsed.mcpServers.github_ci.env.GITHUB_TOKEN).toBe(
+        "forgejo-workflow-token",
+      );
+
+      delete process.env.DEFAULT_WORKFLOW_TOKEN;
+    });
+
+    test("should not call checkActionsReadPermission on Forgejo", async () => {
+      process.env.DEFAULT_WORKFLOW_TOKEN = "forgejo-workflow-token";
+      fetchSpy.mockClear();
+
+      await prepareMcpConfig({
+        githubToken: "test-token",
+        owner: "test-owner",
+        repo: "test-repo",
+        branch: "test-branch",
+        baseBranch: "main",
+        allowedTools: [],
+        mode: "tag",
+        context: mockPRContext,
+      });
+
+      // fetch should not have been called — the permission check is skipped
+      expect(fetchSpy).not.toHaveBeenCalled();
+
+      delete process.env.DEFAULT_WORKFLOW_TOKEN;
+    });
+
+    test("should keep server names as github_* on Forgejo", async () => {
+      process.env.DEFAULT_WORKFLOW_TOKEN = "forgejo-workflow-token";
+
+      const result = await prepareMcpConfig({
+        githubToken: "test-token",
+        owner: "test-owner",
+        repo: "test-repo",
+        branch: "test-branch",
+        baseBranch: "main",
+        allowedTools: [],
+        mode: "tag",
+        context: {
+          ...mockPRContext,
+          inputs: { ...mockPRContext.inputs, useCommitSigning: true },
+        },
+      });
+
+      const parsed = JSON.parse(result);
+      // Server names must stay as github_* to match hardcoded tool names
+      expect(parsed.mcpServers.github_comment).toBeDefined();
+      expect(parsed.mcpServers.github_file_ops).toBeDefined();
+      expect(parsed.mcpServers.github_ci).toBeDefined();
+
+      delete process.env.DEFAULT_WORKFLOW_TOKEN;
+    });
+
+    test("should include comment server on Forgejo in tag mode", async () => {
+      const result = await prepareMcpConfig({
+        githubToken: "test-token",
+        owner: "test-owner",
+        repo: "test-repo",
+        branch: "test-branch",
+        baseBranch: "main",
+        allowedTools: [],
+        mode: "tag",
+        context: mockContext,
+      });
+
+      const parsed = JSON.parse(result);
+      expect(parsed.mcpServers.github_comment).toBeDefined();
+      expect(parsed.mcpServers.github_comment.env.GITHUB_TOKEN).toBe(
+        "test-token",
+      );
+    });
+
+    test("should include inline comment server on Forgejo for PRs", async () => {
+      const result = await prepareMcpConfig({
+        githubToken: "test-token",
+        owner: "test-owner",
+        repo: "test-repo",
+        branch: "test-branch",
+        baseBranch: "main",
+        allowedTools: [
+          "mcp__github_inline_comment__create_inline_comment",
+        ],
+        mode: "tag",
+        context: mockPRContext,
+      });
+
+      const parsed = JSON.parse(result);
+      expect(parsed.mcpServers.github_inline_comment).toBeDefined();
+      expect(
+        parsed.mcpServers.github_inline_comment.env.GITHUB_TOKEN,
+      ).toBe("test-token");
+    });
+  });
 });