fix(audit): require integer rolling retention keep limits

Validate keepPerFamily/--keep-per-family as non-negative integers, remove silent flooring, add regression coverage, and sync runbook/docs wording.

docs/plans/state.json

@@ -475,6 +475,24 @@
       ],
       "test_status": "pnpm test:run src/audit/phase0BaselineSummary.test.ts + pnpm typecheck passing"
     },
+    "phase0-live-baseline-retention-keep-integer-validation": {
+      "status": "completed",
+      "date": "2026-02-27",
+      "updated": "2026-02-27",
+      "summary": "Hardened rolling retention keep-limit handling by requiring non-negative integer `keepPerFamily` values in both planner and prune CLI (`--keep-per-family`), eliminating silent flooring of fractional values and surfacing invalid operator input early.",
+      "files_modified": [
+        "src/audit/phase0BaselineArtifactRetention.ts",
+        "src/audit/phase0BaselineArtifactRetention.test.ts",
+        "scripts/prune-phase0-baseline-artifacts.ts",
+        "README.md",
+        "docs/api/PROTOCOL.md",
+        "docs/architecture/AGENT_DIAGRAM.md",
+        "docs/architecture/GATEWAY_SESSIONS_AND_QUEUE.md",
+        "docs/plans/2026-02-25-phase0-instrumentation-ticket-checklist.md",
+        "docs/plans/state.json"
+      ],
+      "test_status": "pnpm test:run src/audit/phase0BaselineArtifactRetention.test.ts + pnpm typecheck passing"
+    },
     "phase0-instrumentation-ticket-checklist": {
       "status": "completed",
       "date": "2026-02-25",