[codex] add real repository quickstart

[eliottreich]

Summary

• add a five-minute tutorial using the public TaskBounty Check repository as a real example
• include a pinned copy-paste dry run and its verified 0.1.6 output
• explain the exact data boundary, CI setup, agent/MCP use, limitations, and next steps
• link the canonical tutorial from the README

Verification

• tutorial command was run against the published npm 0.1.6 package
• observed result: 1 repo, 2 workflow files, 0 maintenance candidates, 0 private-review items
• markdown passes git diff --check
• no package or runtime code changes