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/workflows/unit-tests.yml

@@ -191,7 +191,7 @@ jobs:
         run: bash tests/infrastructure/scripts/preflight-cleanup.test.sh
 
       # The pin lives in two files; split-brain is exactly the unexplained
-      # breakage D017 exists to prevent, so assert they agree.
+      # breakage ADR 0022 exists to prevent, so assert they agree.
       - name: Pester pin agrees across workflow and image
         run: |
           wf=$(sed -n 's/^  PESTER_VERSION: *//p' .github/workflows/unit-tests.yml)