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/package-currency.yml

@@ -1,6 +1,6 @@
 name: PVE Package Currency
 
-# Lane 2 of the two-lane CI split (see docs/lane2-change-plan.md).
+# Lane 2 of the two-lane CI split (see docs/decisions/0017-ci-runs-two-lanes-a-pinned-gating-lane-and-a-report-only-currency-lane.md).
 #
 # Lane 1 (integration-tests.yml) pins the nested nodes to what the ISO ships
 # and never upgrades — that pin is what makes it a stable merge gate.

.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)