fix(prd-7): M3 security review — self-trigger guard, actor permissions, trigger bypass, platform validation

- Add self-mention/bot loop prevention in actor.ts (blocks actor matching bot_name)
- Switch Forgejo permission check to actor-level via getCollaboratorPermissionLevel
  with fallback to token-level repos.get() when collaborator API returns 403
- Close empty-body PR review comment trigger bypass (now returns false instead of true)
- Add platform detection cross-validation (reject PLATFORM=forgejo on github.com)
- Document URL builder defense-in-depth rationale in comment-logic.ts
- Update/add tests across 6 test files (actor, permissions, trigger, platform, mcp, m7)

Progress: 50% complete (3/6 milestones) — next: M4 test coverage review

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

test/permissions.test.ts

@@ -316,6 +316,8 @@ describe("checkWritePermissions", () => {
       savedForgejo = process.env.FORGEJO;
       savedServerUrl = process.env.GITHUB_SERVER_URL;
       process.env.PLATFORM = "forgejo";
+      // Ensure cross-validation doesn't reject PLATFORM=forgejo
+      process.env.GITHUB_SERVER_URL = "https://git.example.com";
     });
 
     afterEach(() => {
@@ -329,25 +331,75 @@ describe("checkWritePermissions", () => {
       _resetPlatformCache();
     });
 
-    const createForgejoOctokit = (permissions: {
+    // Helper: Forgejo octokit where collaborator API works (actor-level check)
+    const createForgejoActorOctokit = (permission: string) => {
+      return {
+        repos: {
+          getCollaboratorPermissionLevel: async () => ({
+            data: { permission },
+          }),
+          get: async () => {
+            throw new Error("Should not fall back to repos.get");
+          },
+        },
+      } as any;
+    };
+
+    // Helper: Forgejo octokit where collaborator API fails (falls back to token-level)
+    const createForgejoFallbackOctokit = (permissions: {
       admin?: boolean;
       push?: boolean;
       pull?: boolean;
     }) => {
       return {
         repos: {
+          getCollaboratorPermissionLevel: async () => {
+            throw new Error("403 Forbidden");
+          },
           get: async () => ({
             data: { permissions },
           }),
-          getCollaboratorPermissionLevel: async () => {
-            throw new Error("Should not be called on Forgejo");
-          },
         },
       } as any;
     };
 
-    test("should return true when Forgejo token has push access", async () => {
-      const mockOctokit = createForgejoOctokit({
+    test("should use actor-level permission check on Forgejo (write)", async () => {
+      const mockOctokit = createForgejoActorOctokit("write");
+      const context = createContext();
+
+      const result = await checkWritePermissions(mockOctokit, context);
+
+      expect(result).toBe(true);
+      expect(coreInfoSpy).toHaveBeenCalledWith(
+        expect.stringContaining(
+          "Forgejo actor 'test-user' permission level: write",
+        ),
+      );
+    });
+
+    test("should use actor-level permission check on Forgejo (admin)", async () => {
+      const mockOctokit = createForgejoActorOctokit("admin");
+      const context = createContext();
+
+      const result = await checkWritePermissions(mockOctokit, context);
+
+      expect(result).toBe(true);
+    });
+
+    test("should reject actor with read-only permission on Forgejo", async () => {
+      const mockOctokit = createForgejoActorOctokit("read");
+      const context = createContext();
+
+      const result = await checkWritePermissions(mockOctokit, context);
+
+      expect(result).toBe(false);
+      expect(coreWarningSpy).toHaveBeenCalledWith(
+        expect.stringContaining("insufficient permissions: read"),
+      );
+    });
+
+    test("should fall back to token-level check when collaborator API fails", async () => {
+      const mockOctokit = createForgejoFallbackOctokit({
         admin: false,
         push: true,
         pull: true,
@@ -357,26 +409,16 @@ describe("checkWritePermissions", () => {
       const result = await checkWritePermissions(mockOctokit, context);
 
       expect(result).toBe(true);
+      expect(coreWarningSpy).toHaveBeenCalledWith(
+        expect.stringContaining("Falling back to token-level permission check"),
+      );
       expect(coreInfoSpy).toHaveBeenCalledWith(
-        expect.stringContaining("Forgejo token has write access"),
+        expect.stringContaining("token-level fallback"),
       );
     });
 
-    test("should return true when Forgejo token has admin access", async () => {
-      const mockOctokit = createForgejoOctokit({
-        admin: true,
-        push: false,
-        pull: true,
-      });
-      const context = createContext();
-
-      const result = await checkWritePermissions(mockOctokit, context);
-
-      expect(result).toBe(true);
-    });
-
-    test("should return false when Forgejo token has only pull access", async () => {
-      const mockOctokit = createForgejoOctokit({
+    test("should return false on fallback when token has only pull access", async () => {
+      const mockOctokit = createForgejoFallbackOctokit({
         admin: false,
         push: false,
         pull: true,
@@ -386,14 +428,14 @@ describe("checkWritePermissions", () => {
       const result = await checkWritePermissions(mockOctokit, context);
 
       expect(result).toBe(false);
-      expect(coreWarningSpy).toHaveBeenCalledWith(
-        expect.stringContaining("insufficient permissions"),
-      );
     });
 
-    test("should return false when permissions object is missing", async () => {
+    test("should return false on fallback when permissions object is missing", async () => {
       const mockOctokit = {
         repos: {
+          getCollaboratorPermissionLevel: async () => {
+            throw new Error("403 Forbidden");
+          },
           get: async () => ({ data: {} }),
         },
       } as any;
@@ -407,32 +449,8 @@ describe("checkWritePermissions", () => {
       );
     });
 
-    test("should not call getCollaboratorPermissionLevel on Forgejo", async () => {
-      let collaboratorCalled = false;
-      const mockOctokit = {
-        repos: {
-          get: async () => ({
-            data: { permissions: { admin: false, push: true, pull: true } },
-          }),
-          getCollaboratorPermissionLevel: async () => {
-            collaboratorCalled = true;
-            return { data: { permission: "write" } };
-          },
-        },
-      } as any;
-      const context = createContext();
-
-      await checkWritePermissions(mockOctokit, context);
-
-      expect(collaboratorCalled).toBe(false);
-    });
-
     test("should still bypass for allowed_non_write_users on Forgejo", async () => {
-      const mockOctokit = createForgejoOctokit({
-        admin: false,
-        push: false,
-        pull: true,
-      });
+      const mockOctokit = createForgejoActorOctokit("read");
       const context = createContext();
 
       const result = await checkWritePermissions(