docs: one purpose per docs folder, lower-case names, archive point-in-time material

[pacphi]

Summary

Implements Phase 1 and Phase 2 of the documentation taxonomy plan.

• adds tested relocation and layout guards
• moves 123 docs, lower-cases guide names, homes active plans and dormant proposals
• indexes all living guides and every newly archived file
• updates repository-only documentation rules and reference paths

Validation

• relocation dry run: would move 123; 369 link(s) in 102 file(s); 259 mention line(s)
• internal lychee: 0 errors
• markdownlint: 0 issues
• focused documentation suites: 52 passing
• full unit suite: all available tests pass; the only two failures are ui-chrome-launch because this isolated worktree has no playwright dependency
• archive-only lychee: exactly 12 known historical errors, deferred to Phase 3 after this PR merges

Package preview

npm pack --dry-run lists lower-case shipped docs and no docs/evidence/ paths.

GitHub does not redirect renamed files, so older links to all-caps documentation URLs, including the shipped ak code reference, resolve only on older tags. blob/main links to the new paths resolve after merge.

Phase 3 is intentionally sequenced after this merge on docs/archive-link-repair.