fix(ci): add missing SPDX headers to workflow files (#72)

The `Governance` workflow fails on every push here. Its **Workflow
security linter** job requires every `.github/workflows/*.yml` to carry
an `SPDX-License-Identifier` comment on **line 1**:

```
ERROR: .github/workflows/<file> missing SPDX header
Add SPDX header + permissions:
```

This is a real gate catching a real gap — the files below never had the
header. Each failure is another red workflow feeding the `ci_activity`
notification flood.

### Licence identifier

`MPL-2.0` — taken from **this repo's own existing workflow headers**,
which agree unanimously. It is *not* assumed or copied from a template.
Repos with mixed or absent identifiers were deliberately skipped for a
manual decision rather than guessed, because an earlier estate sweep
flattened co-developed AGPL repos to MPL-2.0.

### Files

`codeql.yml generator-generic-ossf-slsa3-publish.yml pages.yml php.yml
rust.yml`

Pure one-line prepend — no other content is touched.

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

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/codeql.yml

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: MPL-2.0
 # For most projects, this workflow file will not need changing; you simply need
 # to commit it to your repository.
 #

.github/workflows/generator-generic-ossf-slsa3-publish.yml

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: MPL-2.0
 # This workflow uses actions that are not certified by GitHub.
 # They are provided by a third-party and are governed by
 # separate terms of service, privacy policy, and support

.github/workflows/pages.yml

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: MPL-2.0
 name: GitHub Pages (Ddraig SSG)
 on:
   push:

.github/workflows/php.yml

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: MPL-2.0
 name: PHP Composer
 
 on:

.github/workflows/rust.yml

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: MPL-2.0
 name: Rust
 
 on: