docs: prepare public repository release

[randomsnowflake]

Summary

• Add public-facing project docs: CONTRIBUTING.md, SECURITY.md, CHANGELOG.md, and docs/publication.md.
• Add GitHub Actions CI for the offline pytest suite across Python 3.9-3.13.
• Refresh the README with CI/license badges, current test count, public docs links, and the public-ready project layout.
• Make the bundled Hermes skill install cleanly by moving its linked example into references/ and adding Hermes/uv-safe dependency guidance.

Public release readiness

• Required root docs are now present: README, LICENSE, CONTRIBUTING, SECURITY, CHANGELOG, .env.example.
• Publication checklist documents final visibility-flip checks and explicitly separates this PR from the actual GitHub visibility change.
• Examples/tests remain offline and use fictional data/placeholders only.
• Send safety remains prominent: preview is free, send is chargeable, ambiguous send retries are dangerous.

Verification

• pytest -q → 48 passed in 0.14s
• python -m build → sdist + wheel built successfully
• Hermes installed-skill smoke: copied skill/briefklick-send-letter into ~/.hermes/skills/integrations/briefklick-send-letter
• skill_view('briefklick-send-letter') loads and exposes references/natural_language_to_code.md
• Hermes-style uv execution from outside the repo: uv run --with /home/clawdbot/work/briefklick-python /tmp/briefklick_hermes_smoke.py → mocked preview→send flow passed
• Focused current-tree secret scan → no findings outside the publication scanner text
• Large blob scan → no blobs >1MB
• README/docs/skill local link check → ok

Notes

The repository is still private. After this PR is reviewed and merged, a maintainer should flip visibility separately in GitHub settings after one final local checkout audit.