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.

tests/PSProxmoxVE.Tests/Integration/16_Cluster.Tests.ps1

@@ -112,7 +112,7 @@ Describe 'Cluster Config & HA Lifecycle — Integration' -Tag 'Integration' {
             $result | Should -Not -BeNullOrEmpty
             $script:ClusterCreated = $true
 
-            # New-PveCluster -Wait returns only once the cluster is quorate (D014),
+            # New-PveCluster -Wait returns only once the cluster is quorate (ADR 0014),
             # which is what makes the join below safe without a sleep.
             $cluster = @(Get-PveClusterStatus -ErrorAction Stop) |
                 Where-Object { $_.Type -eq 'cluster' } | Select-Object -First 1