feat: add build info to /api/health for deployment verification

- Return commitSha, commitMessage, and deploymentUrl from Vercel env vars
- Document verification workflow in CLAUDE.md

apps/web/src/app/api/health/route.ts

@@ -18,6 +18,12 @@ export async function GET(request: NextRequest) {
   return NextResponse.json({
     status: 'ok',
     timestamp: new Date().toISOString(),
+    build: {
+      // Vercel provides these at runtime
+      commitSha: process.env.VERCEL_GIT_COMMIT_SHA?.slice(0, 7) || 'local',
+      commitMessage: process.env.VERCEL_GIT_COMMIT_MESSAGE || 'local',
+      deploymentUrl: process.env.VERCEL_URL || 'localhost',
+    },
     env: {
       hasDatabase: !!process.env.DATABASE_URL,
       nodeEnv: process.env.NODE_ENV,