fix: use repos.getBranch() on Forgejo for branch SHA lookup

Forgejo uses /git/refs (plural) not /git/ref (singular), causing
git.getRef() to 404. Use repos.getBranch() on Forgejo instead,
which returns the same SHA data via a compatible endpoint.

Also adds the tag mode workflow (.forgejo/workflows/claude.yml)
for @claude mentions on issues, PR reviews, and label triggers.

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

src/github/operations/branch.ts

@@ -13,6 +13,7 @@ import type { GitHubPullRequest } from "../types";
 import type { Octokits } from "../api/client";
 import type { FetchDataResult } from "../data/fetcher";
 import { generateBranchName } from "../../utils/branch-template";
+import { detectPlatform } from "../../platform/detector";
 
 /**
  * Extracts the first label from GitHub data, or returns undefined if no labels exist
@@ -204,14 +205,25 @@ export async function setupBranch(
   let sourceSHA: string | undefined;
 
   try {
-    // Get the SHA of the source branch to verify it exists
-    const sourceBranchRef = await octokits.rest.git.getRef({
-      owner,
-      repo,
-      ref: `heads/${sourceBranch}`,
-    });
-
-    sourceSHA = sourceBranchRef.data.object.sha;
+    // Get the SHA of the source branch to verify it exists.
+    // Forgejo uses /git/refs (plural) not /git/ref (singular), so
+    // git.getRef() 404s. Use repos.getBranch() on Forgejo instead.
+    const platform = detectPlatform();
+    if (platform === "forgejo") {
+      const branchResponse = await octokits.rest.repos.getBranch({
+        owner,
+        repo,
+        branch: sourceBranch,
+      });
+      sourceSHA = branchResponse.data.commit.sha;
+    } else {
+      const sourceBranchRef = await octokits.rest.git.getRef({
+        owner,
+        repo,
+        ref: `heads/${sourceBranch}`,
+      });
+      sourceSHA = sourceBranchRef.data.object.sha;
+    }
     console.log(`Source branch SHA: ${sourceSHA}`);
 
     // Extract first label from GitHub data