ci: Secret Scanner caller must grant the reusable's job permissions (#177)

Adds `pull-requests: write` + `actions: read` to the `scan` job so the
standards secret-scanner reusable's gitleaks job can start. A called
workflow cannot exceed the caller's grant; without this the run dies at
startup (`startup_failure`, 0s) — the estate-wide Secret Scanner red.

Mechanical caller-side fix, mirrors hyperpolymath/standards#472. No
behaviour change beyond letting the scan run.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: hyperpolymath

.github/workflows/secret-scanner.yml

@@ -15,5 +15,9 @@ permissions:
 
 jobs:
   scan:
+    permissions:
+      contents: read
+      pull-requests: write
+      actions: read
     uses: hyperpolymath/standards/.github/workflows/secret-scanner-reusable.yml@7c9db0e5909aab77bf11a444ade3b95c1d2b702e
-    secrets: inherit
+    secrets: inherit