Suppress existing auto PR sync status

Summary

• Skip auto-PR creation when session PR metadata already exists.
• Keep stale metadata repair silent when auto-PR only finds an existing PR.
• Add workflow tests for both skip paths.

Tests

• bun test "apps/web/app/workflows/chat.test.ts"
• bun run check "apps/web/app/workflows/chat.ts" "apps/web/app/workflows/chat.test.ts"
• turbo typecheck --filter=web