chore(governance): add CC BY 4.0 license

[stacknil]

Summary

• Add an explicit CC BY 4.0 license for original written content and pattern cards.
• Exclude third-party platform material, quotations, and linked resources from that scope.
• Remove only the stale repo.required_file_missing baseline entry for LICENSE.
• Record the clean-worktree consumer audit and keep the remote gate non-blocking.

Design decision

sec-writeups-public is intentionally a public, reusable pattern library. CC BY 4.0 is applied to original written content and pattern cards, while third-party material remains governed by its own terms. The baseline cleanup is a targeted policy-state update, not a bulk refresh.

Main risk

Readers could incorrectly assume the repository license covers third-party TryHackMe or linked material. The scope is repeated in LICENSE, README.md, and POLICY.md to keep that boundary explicit.

Compatibility impact

• The default repo-sentinel required-file policy is now satisfied.
• No note, pattern, taxonomy, placeholder, or scanner behavior changed.
• The baseline audit remains non-blocking; no changed-file remote gate is enabled.

Rollback path

Revert the two commits on this branch. That removes the license and restores the prior baseline state; the missing-license governance condition must then remain explicitly unresolved rather than suppressed.

Evidence and validation

• Consumer commit audited: 1dff619841eeaa136f12897db39064ca43a2a65f
• Local scanner: repo-sentinel 0.8.0 (pre-release/local evidence, not a formal remote pin)
• Exact command: repo-sentinel baseline audit --format json --baseline .reposentinel-baseline.json .
• Audit artifact SHA-256: ce5e84b0bff3c8825c51141a9278612837ae7cbf80baaafd52b1c1a71167fe9a
• Audit result: repo.required_file_missing=0, stale=0, exit 0
• python scripts/render_tags_doc.py --check — passed
• python -m unittest discover -s tests -p 'test_*.py' — 6 passed
• python scripts/check_markdown.py — passed for 157 Markdown files
• python -m pre_commit run --files .reposentinel-baseline.json LICENSE POLICY.md README.md docs/repo-sentinel-baseline-review.md — passed

Formal v0.8.0 release pinning, synthetic pass/fail/redaction tests, changed-file gate rollout, and issue #5 closure remain follow-up work.