ci: Secret Scanner caller must grant the reusable's job permissions (#181)

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

@@ -13,6 +13,10 @@ permissions:
   contents: read
 jobs:
   scan:
+    permissions:
+      contents: read
+      pull-requests: write
+      actions: read
     uses: hyperpolymath/standards/.github/workflows/secret-scanner-reusable.yml@d135b05bfc647d0c0fbfedc7e80f37ea50f49236
     timeout-minutes: 10
     secrets: inherit
@@ -25,4 +29,4 @@ jobs:
       - name: TruffleHog Secret Scan
         uses: trufflesecurity/trufflehog@d411fff7b8879a62509f3fa98c07f247ac089a51 # v3.95.5
         with:
-          extra_args: --only-verified --fail
+          extra_args: --only-verified --fail