feat(security): reject secret last-applied annotations (CUST-WP-0073)

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a0e7fb-275d-7db0-b1df-39ed943427da

argocd/platform-addons/secret-annotation-guard/README.md

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+# Secret annotation guard
+
+Implemented under the existing CUST-WP-0073-T03; no new platform workplan.
+Native admission policy denies the last-applied annotation on Secret CREATE
+and UPDATE, including an empty value. No new workload or controller.
+
+Before the first manual sync, run `scripts/secret_annotation_maintenance.py`
+on railiance01 through the supervised admin path, first without arguments,
+then with `--clean`. It removes only the duplicate annotation; it never prints
+kubectl output or Secret values. Concurrent Secret churn can stop cleanup;
+inspect the receipt before retrying. Preserve only names, counts and booleans.
+
+Declare the policy through the pinned `secret-annotation-guard` Argo Application;
+automated sync and prune are off. Platform-addons AppProject must allow both
+admission kinds. Sync policy first and inspect typeChecking; bind only after
+cleanup. Test clean CREATE/UPDATE and denied annotated CREATE/UPDATE using
+synthetic data in whitehat; verify client-side apply is denied, then delete
+only the test fixture. Secret writers must use server-side apply or replace.
+
+Rollback is a manual Argo sync of a reviewed revision without the binding
+(with explicit resource-scoped pruning), or attended break-glass deletion of
+the binding followed by Git reconciliation. Removing the binding reopens this
+leak path. The policy does not rotate credentials or isolate agent accounts.