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/trigger-validation.test.ts

@@ -2,7 +2,8 @@ import {
   checkContainsTrigger,
   escapeRegExp,
 } from "../src/github/validation/trigger";
-import { describe, it, expect } from "bun:test";
+import { describe, it, expect, beforeEach, afterEach } from "bun:test";
+import { _resetPlatformCache } from "../src/platform/detector";
 import {
   createMockContext,
   mockIssueAssignedContext,
@@ -451,6 +452,75 @@ describe("checkContainsTrigger", () => {
   });
 });
 
+describe("Forgejo empty-body trigger bypass", () => {
+  let savedPlatform: string | undefined;
+  let savedServerUrl: string | undefined;
+
+  beforeEach(() => {
+    _resetPlatformCache();
+    savedPlatform = process.env.PLATFORM;
+    savedServerUrl = process.env.GITHUB_SERVER_URL;
+    process.env.PLATFORM = "forgejo";
+    process.env.GITHUB_SERVER_URL = "https://git.example.com";
+  });
+
+  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;
+    _resetPlatformCache();
+  });
+
+  it("should NOT trigger when Forgejo PR review comment has no body", () => {
+    // Previously this returned true unconditionally, bypassing trigger check
+    const context = {
+      ...mockPullRequestReviewCommentContext,
+      payload: {
+        ...mockPullRequestReviewCommentContext.payload,
+        comment: {
+          ...(mockPullRequestReviewCommentContext.payload as any).comment,
+          body: "",
+        },
+      },
+    } as ParsedGitHubContext;
+    expect(checkContainsTrigger(context)).toBe(false);
+  });
+
+  it("should NOT trigger when Forgejo PR review comment body is missing", () => {
+    const context = {
+      ...mockPullRequestReviewCommentContext,
+      payload: {
+        ...mockPullRequestReviewCommentContext.payload,
+        comment: {
+          ...(mockPullRequestReviewCommentContext.payload as any).comment,
+          body: undefined,
+        },
+      },
+    } as ParsedGitHubContext;
+    expect(checkContainsTrigger(context)).toBe(false);
+  });
+
+  it("should still trigger when Forgejo PR review comment contains trigger phrase", () => {
+    const context = {
+      ...mockPullRequestReviewCommentContext,
+      inputs: {
+        ...mockPullRequestReviewCommentContext.inputs,
+        triggerPhrase: "@claude",
+      },
+      payload: {
+        ...mockPullRequestReviewCommentContext.payload,
+        comment: {
+          ...(mockPullRequestReviewCommentContext.payload as any).comment,
+          body: "@claude please review this line",
+        },
+      },
+    } as ParsedGitHubContext;
+    expect(checkContainsTrigger(context)).toBe(true);
+  });
+});
+
 describe("escapeRegExp", () => {
   it("should escape special regex characters", () => {
     expect(escapeRegExp(".*+?^${}()|[]\\")).toBe(