fix: make serve shutdown reliable

[juliantanx]

Summary

• make aiusage serve shutdown idempotent and register signal handlers once
• close idle connections gracefully, then force-close active connections after a timeout
• add regression coverage for normal, timed-out, and repeated shutdown attempts
• refresh the contributor guide package layout and credit @lmingde for the bug report

Testing

• corepack pnpm --dir packages/cli test (54 files, 418 tests)
• corepack pnpm --filter @juliantanx/aiusage build
• git diff --check

Closes #49