docs: standardize Markdown frontmatter

[salmonumbrella]

Add ordered title, description, and last_edited frontmatter to 23 tracked Markdown documents, including nested READMEs and the Natural Earth provenance note. Preserve document bodies and derive missing dates from the latest body edits in Git history.

Root README.md and AGENTS.md are exempt and retain their original frontmatter state. Keep README unchanged, add the exemption-aware maintenance rule to AGENTS, and preserve CLAUDE.md as its existing symlink to the exempt AGENTS file.

The agent guide requires title, description, and last_edited when creating Markdown files, and a body-edit date update when editing any nonexempt Markdown body. Root README.md and AGENTS.md remain YAML-exempt; metadata-only edits preserve the date.

[roborev-ci]

roborev: Combined Review (950700f)

Verdict: No findings at or above medium severity.

---

Reviewers: 2x codex, codex (security) | Synthesis: codex, 4s | Total: 3m47s

[roborev-ci]

roborev: Combined Review (0e05a45)

Verdict: No findings at or above medium severity.

---

Reviewers: 2x codex, codex (security) | Synthesis: codex | Total: 2m26s

[roborev-ci]

roborev: Combined Review (02af83e)

Verdict: No findings at or above medium severity.

---

Reviewers: 2x codex, codex (security) | Synthesis: codex | Total: 2m15s

[mariusvniekerk]

Closing this PR as rejected. We do not accept formatting-only changes that provide no practical benefit. Git already records change history and timestamps; that is sufficient for our needs.

Please do not submit further PRs of this kind. They create unnecessary review work and will be rejected outright.

^{generated by a clanker}