chore: move internal docs into docs/ (#42)

[johnsoncodehk]

Summary

The compromise agreed in #42: the generated grammar artifacts stay at the repo root (they are the product interface — stable raw URLs), and the internal docs move into docs/, leaving only README.md at the root.

• 6 docs moved via git mv (all R100 renames): COMPLETENESS.md, TESTING.md, TOTAL-PARSING.md, ROADMAP.md, HIGHLIGHT-ISSUES-SURVEY.md, KNOWN-GAPS.md
• Path updates in the machine-maintained ledger chain: test/gap-ledger.ts (--write/--check target), test/gap-issues.ts (ledger read + issue-body references), test/tm-completeness.ts (ledger splice), plus comment/text mentions in test/check.ts, test/tm-mutation.ts
• CI: gap-issues.yml trigger path now docs/KNOWN-GAPS.md; ci.yml comment references updated
• Branch merged with latest master (post-Portable targets: document session + edit API (S1, full-reparse fallback) #61); zero cross-references remain outside docs/

Validation

• Full gate suite 43/43 on the construction branch; on the master-merged tree the affected gates re-verified: gap-ledger --check (docs/KNOWN-GAPS.md is up to date.), gap-ledger-selftest, tm-completeness 97/97 (same count as master), tsc --noEmit clean
• git show --name-status -M: all six moves are R100
• Residual-reference sweep (rg outside docs/): zero hits