fix(ci): repoint codeql-action at a SHA that exists (#36)

`github/codeql-action@29b1f65c1f735799893313399435a59f54045865` is
pinned here but **exists in no repository** — the GitHub API returns 422
for it.

CodeQL therefore could not start: the run graph fails to build and the
job reports `startup_failure`, so this repository has had **no CodeQL
scanning at all**.

Repointed at `4187e74d05793876e9989daffde9c3e66b4acd07`, which is what
the `v3` tag currently resolves to (v3.37.3), verified against the API.

Found while auditing the estate: the same non-existent SHA was pinned in
**104 repositories**, so CodeQL was dead across nearly all of them.

----
## Summary by Gitar

- **Security and governance:**
- Added estate-wide `SECURITY.md` policy and removed duplicate
governance documentation
- Updated workflow permissions across multiple CI configurations for
security compliance

<sub>This will update automatically on new commits.</sub>

---------

Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: hyperpolymath

SECURITY.md

@@ -1,5 +1,19 @@
 # Security Policy
 
+TEMPLATE INSTRUCTIONS (delete this block before publishing)
+Replace all {{PLACEHOLDER}} values with your information:
+  Squisher Corpus     - Your project name
+  hyperpolymath            - GitHub username or org (e.g., hyperpolymath)
+  squisher-corpus             - Repository name
+  j.d.a.jewell@open.ac.uk   - Security contact email
+  TBD  - Your PGP key fingerprint (40 chars, no spaces)
+  https://github.com/hyperpolymath.gpg      - URL to your public PGP key
+  github.com/hyperpolymath/squisher-corpus          - Your website/domain
+  2026     - Current year for copyright
+
+Optional: Remove sections that don't apply (e.g., PGP if you don't use it)
+-->
+
 We take security seriously. We appreciate your efforts to responsibly disclose vulnerabilities and will make every effort to acknowledge your contributions.
 
 ## Table of Contents