Drop third-party apt sources before the video job's ffmpeg install

The video full-clip job needs ffmpeg from Ubuntu's own repositories, but its
apt-get update refreshed every source on the runner image, so a broken
third-party index failed the job twice on 2026-09-09: google-chrome.list
served a Hash Sum mismatch (bc1428ab... against 233e56de...) and both the run
and its rerun died at Install ffmpeg with ten otherwise-green jobs around
them. Remove every non-Ubuntu source definition from
/etc/apt/sources.list.d before updating, so no vendor's broken index can
fail a step that never needed it. The only apt-get in the workflows.

pre-commit: 1) maintain.sh - exit 2 at the uv-secure gate, the three triaged
no-fix advisories in docs/development.md (rechecked 2026-09-09 against PyPI);
ruff check+format clean, pyright src/ 0 errors, 2368 passed 3 skipped run
separately, same gate run as the preceding commit with no post-gate edit
between them; 2) /simplify - single pass, no fan-out available, no findings;
3) docs sync - grepped apt-get/google-chrome/Install ffmpeg across docs/,
README, .claude/rules/, tests/: no other references, the rationale lives in
the step's comment; 4) CLAUDE.md - audited against the structure rule, no
additions, revise-claude-md: no additions

.github/workflows/test.yml

@@ -94,7 +94,15 @@ jobs:
         with:
           python-version: "3.12"
       - name: Install ffmpeg
-        run: sudo apt-get update && sudo apt-get install --yes ffmpeg
+        # Third-party apt sources baked into the runner image (google-chrome et
+        # al.) can publish a broken index and fail `apt-get update`, while this
+        # job only needs ffmpeg from Ubuntu's own repositories: drop every
+        # non-Ubuntu source definition before updating.
+        run: |
+          sudo find /etc/apt/sources.list.d -mindepth 1 -maxdepth 1 \
+            ! -name 'ubuntu.sources' -delete
+          sudo apt-get update
+          sudo apt-get install --yes ffmpeg
       - name: Sync dev environment
         run: uv sync --frozen --extra dev
       - name: Run full-clip video test