ci(deps): pin GitHub Actions to commit SHAs (#8065)

Every third-party GitHub Action in the workflows is now pinned to a full
commit SHA instead of a floating major tag, closing a supply-chain gap
where a mutable tag like `@v6` could be force-pushed to point at
malicious code. Seven actions were already SHA-pinned; this brings the
remaining first-party `actions/*` and `astral-sh/setup-uv` references in
line.

## Changes
- Pinned all previously tag-referenced actions to their current commit
SHA with the exact release as a trailing comment (the format Dependabot
reads and updates): `actions/checkout` → `v6.0.3`,
`actions/setup-python` → `v6.2.0`, `actions/github-script` → `v9.0.0`,
`actions/upload-artifact` → `v7.0.1`, `actions/download-artifact` →
`v8.0.1`, `actions/cache/{restore,save}` → `v5.0.5`,
`actions/configure-pages` → `v6.0.0`, `actions/deploy-pages` → `v5.0.0`,
`actions/upload-pages-artifact` → `v5.0.0`,
`actions/create-github-app-token` → `v3.2.0`, and `astral-sh/setup-uv` →
`v7.6.0`.
- Applied across all workflow files plus the `uv_setup` composite
action; local same-repo references (`./.github/workflows/_*.yml`,
`./.github/actions/uv_setup`) left as path refs since they resolve from
the same commit.

.github/workflows/reopen_on_assignment.yml

@@ -26,7 +26,7 @@ jobs:
 
     steps:
       - name: Find and reopen matching PRs
-        uses: actions/github-script@v9
+        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
         with:
           script: |
             const { owner, repo } = context.repo;