ci: gate optional Claude and security-scan jobs behind repository variables

Add per-job `if:` gates so deployments that don't want or can't run these
jobs can switch them off without editing workflows:

- ENABLE_CLAUDE_CODE gates the Claude jobs (interactive @claude, the
  CLAUDE.md audit, and the REVIEW.md drift audit).
- ENABLE_WORKFLOW_SECURITY_SCAN gates the Zizmor job, which uploads SARIF
  and so needs GitHub code scanning enabled.

Both default to enabled: a job runs unless its variable is explicitly set
to 'false', so existing behaviour is unchanged where the variables are unset.

Author: d-cs

.github/workflows/check-review-md.yml

@@ -14,7 +14,10 @@ concurrency:
 
 jobs:
   audit:
+    # Set the ENABLE_CLAUDE_CODE repository variable to 'false' to turn off Claude
+    # jobs; leave it unset (the default) to keep them enabled.
     if: >-
+      vars.ENABLE_CLAUDE_CODE != 'false' &&
       github.event.pull_request.draft == false &&
       github.event.pull_request.head.repo.full_name == github.repository
     runs-on: ubuntu-latest

.github/workflows/claude-md-audit.yml

@@ -15,7 +15,10 @@ concurrency:
 
 jobs:
   audit:
+    # Set the ENABLE_CLAUDE_CODE repository variable to 'false' to turn off Claude
+    # jobs; leave it unset (the default) to keep them enabled.
     if: >-
+      vars.ENABLE_CLAUDE_CODE != 'false' &&
       github.event.pull_request.draft == false &&
       github.event.pull_request.head.repo.full_name == github.repository
     runs-on: ubuntu-latest

.github/workflows/claude.yml

@@ -12,11 +12,16 @@ on:
 
 jobs:
   claude:
+    # Set the ENABLE_CLAUDE_CODE repository variable to 'false' to turn off Claude
+    # jobs; leave it unset (the default) to keep them enabled.
     if: |
-      (github.event_name == 'issue_comment' && contains(github.event.comment.body, '@claude')) ||
-      (github.event_name == 'pull_request_review_comment' && contains(github.event.comment.body, '@claude')) ||
-      (github.event_name == 'pull_request_review' && contains(github.event.review.body, '@claude')) ||
-      (github.event_name == 'issues' && (contains(github.event.issue.body, '@claude') || contains(github.event.issue.title, '@claude')))
+      vars.ENABLE_CLAUDE_CODE != 'false' &&
+      (
+        (github.event_name == 'issue_comment' && contains(github.event.comment.body, '@claude')) ||
+        (github.event_name == 'pull_request_review_comment' && contains(github.event.comment.body, '@claude')) ||
+        (github.event_name == 'pull_request_review' && contains(github.event.review.body, '@claude')) ||
+        (github.event_name == 'issues' && (contains(github.event.issue.body, '@claude') || contains(github.event.issue.title, '@claude')))
+      )
     runs-on: ubuntu-latest
     permissions:
       contents: write

.github/workflows/workflow-checks.yml

@@ -36,6 +36,11 @@ jobs:
 
   zizmor:
     name: Zizmor
+    # Uploads SARIF to the Security tab, which requires GitHub code scanning to be
+    # enabled on the repository. Set the ENABLE_WORKFLOW_SECURITY_SCAN repository
+    # variable to 'false' to skip this job where code scanning isn't available;
+    # leave it unset (the default) to run the scan.
+    if: ${{ vars.ENABLE_WORKFLOW_SECURITY_SCAN != 'false' }}
     runs-on: ubuntu-latest
     permissions:
       security-events: write # Upload SARIF to GitHub Security tab