ci: cut Actions burn — scope push triggers + concurrency-cancel

Workflows fired on bare `[push, pull_request]`, running twice per PR commit
(branch push + PR synchronize). Scope push to [main, master] so PRs fire once;
add concurrency cancel-in-progress (read-only PR checks). Reduces the Actions
minute burn behind the estate spending-limit wall. No SPDX/logic changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/container-policy.yml

@@ -3,7 +3,16 @@ permissions:
   contents: read
 
 name: Container Policy
-on: [push, pull_request]
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+
+# Estate guardrail: scope push to default branches so a PR fires once (not
+# push+PR), and cancel superseded runs. Safe — read-only PR-triggered check.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
   check:
     runs-on: ubuntu-latest