feat(prd-1): implement M7 - entry point + mode handler integration

- Tag mode routes to fetchForgejoData vs fetchGitHubData based on platform
- createInitialComment auto-detects platform for Forgejo spinner/URL format
- update-comment-link uses updateForgejoCommentBody + Forgejo PR compare URL
- update-with-branch uses platform-aware branch/job-run link formatting
- Agent mode needed NO changes (token in run.ts, rest uses Octokit REST)
- 11 new integration tests in test/m7-integration.test.ts
- 785 tests pass, typecheck clean, format clean

Progress: 78% complete (7/9 milestones) - M3, M8, M9 remaining

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

src/modes/tag/index.ts

@@ -12,6 +12,8 @@ import {
   extractOriginalTitle,
   extractOriginalBody,
 } from "../../github/data/fetcher";
+import { fetchForgejoData } from "../../forgejo/data/fetcher";
+import { detectPlatform } from "../../platform/detector";
 import { createPrompt } from "../../create-prompt";
 import { isEntityContext } from "../../github/context";
 import type { GitHubContext } from "../../github/context";
@@ -41,7 +43,7 @@ export async function prepareTagMode({
   // Check if actor is human
   await checkHumanActor(octokit.rest, context);
 
-  // Create initial tracking comment
+  // Create initial tracking comment (auto-detects platform for spinner/URL format)
   const commentData = await createInitialComment(octokit.rest, context);
   const commentId = commentData.id;
 
@@ -49,7 +51,9 @@ export async function prepareTagMode({
   const originalTitle = extractOriginalTitle(context);
   const originalBody = extractOriginalBody(context);
 
-  const githubData = await fetchGitHubData({
+  // Platform-aware data fetching: Forgejo uses REST-only, GitHub uses GraphQL
+  const platform = detectPlatform();
+  const fetchParams = {
     octokits: octokit,
     repository: `${context.repository.owner}/${context.repository.repo}`,
     prNumber: context.entityNumber.toString(),
@@ -60,7 +64,11 @@ export async function prepareTagMode({
     originalBody,
     includeCommentsByActor: context.inputs.includeCommentsByActor,
     excludeCommentsByActor: context.inputs.excludeCommentsByActor,
-  });
+  };
+  const githubData =
+    platform === "forgejo"
+      ? await fetchForgejoData(fetchParams)
+      : await fetchGitHubData(fetchParams);
 
   // Setup branch
   const branchInfo = await setupBranch(octokit, githubData, context);