docs: migrate DECISIONS.md to house-format ADRs, and retire the review folder (#131)

D001-D021 become ADR 0001-0021 in docs/decisions/, one decision per file.
D017's PESTER_VERSION amendment was a second decision in one entry and becomes
ADR 0022. ADR 0023 records the migration and reverses the lane2-change-plan
ruling that deliberately kept DECISIONS.md until the CI lane work landed.

DECISIONS.md is reduced to a stub with a D-to-ADR redirect table, so the four
released CHANGELOG entries and older issue bodies that cite it degrade to a
redirect rather than a dead reference.

docs/review/ and docs/lane2-change-plan.md are deleted (ADR 0024). Of 91
findings, 83 were resolved and six of the seven still open were already GitHub
issues; F021 was the exception and is now #130.

CLAUDE.md's Key Conventions list gains the two rules it was missing and becomes
the checklist, with the ADRs carrying rationale.

.github/copilot-instructions.md

@@ -78,6 +78,5 @@ pve force-cleanup
 
 Before writing new code, read these files to understand established patterns and open issues:
 
-- [`DECISIONS.md`](../DECISIONS.md) — architectural decisions and anti-patterns that must not be reintroduced.
-- [`docs/review/findings.json`](../docs/review/findings.json) — stable findings database (IDs F001… are permanent; resolved findings are marked, never deleted).
-- [`docs/review/REVIEW_REPORT.md`](../docs/review/REVIEW_REPORT.md) — latest full review report.
+- [`docs/decisions/`](../docs/decisions/) — architectural decision records, one per file: what was chosen, what was rejected, and why. The rules themselves are the "Key Conventions" list in [`CLAUDE.md`](../CLAUDE.md).
+- The repository's open GitHub issues — all outstanding work is tracked there.