[codex] Stop tracking runtime data files

[Dreamstore2046]

Summary

• stop tracking data/orders.json and data/spending.json while keeping local copies available to developers
• add data/*.json ignore coverage, data/.gitkeep, and an ADR for the local-data persistence decision
• add a PR metadata guard that blocks runtime JSON files from being added or modified again without blocking this cleanup deletion
• document data/ as machine-local runtime state in the README and data README

Validation

• git check-ignore -v data/orders.json data/spending.json
• git ls-files data
• git diff --cached --check
• Verified the guard diff filter excludes deletions while blocking added/copied/modified/renamed data/*.json files.

Closes #26.

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