fix(prd-7): M2 error handling review — fix process.exit bypasses, TOCTOU gap, URL consistency

- F1: Replace process.exit(1) with throw in branch.ts (cleanup was bypassed)
- F2: Replace process.exit(1) with throw in install-mcp-server.ts (same issue)
- F2b: Replace process.exit(1) with throw in create-prompt/index.ts (same issue)
- F3: Use getServerUrl() instead of GITHUB_SERVER_URL constant in branch-cleanup.ts
- F4: Use updated_at for review TOCTOU filtering in Forgejo data fetcher
- Accepted notes: no pagination >100, no retry, unconditional trigger, image no-op

Progress: 33% complete — M2 done, M3 (security review) next

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

src/mcp/install-mcp-server.ts

@@ -241,7 +241,6 @@ export async function prepareMcpConfig(
     // User's config will be passed as separate --mcp-config flags
     return JSON.stringify(baseMcpConfig, null, 2);
   } catch (error) {
-    core.setFailed(`Install MCP server failed with error: ${error}`);
-    process.exit(1);
+    throw new Error(`Install MCP server failed with error: ${error}`);
   }
 }