Make action self-contained: install ffmpeg and Playwright internally after trigger check

[DeDuckProject]

Background

Currently, users who adopt git-glimpse must pre-install ffmpeg and Playwright
Chromium in their workflow before the action runs:

- name: Install FFmpeg
  run: sudo apt-get install -y ffmpeg

- name: Install Playwright Chromium
  run: npx playwright install chromium --with-deps

- uses: DeDuckProject/git-glimpse@v1
  ...

As a workaround, we now ship a separate
check-trigger action that users can run
before their installs to skip them when the pipeline won't execute. But this
still requires users to structure their workflow correctly and understand the
pattern.

Proposed improvement

Move the ffmpeg and Playwright installs into packages/action/src/index.ts,
running them only after evaluateTrigger() returns shouldRun = true. The
action becomes fully self-contained — users need no pre-install steps at all:

# User workflow becomes just:
- uses: actions/checkout@v4
  with:
    fetch-depth: 0

- uses: DeDuckProject/git-glimpse@v1
  with:
    config-path: git-glimpse.config.ts
  env:
    ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

The action would handle:

1. Evaluate trigger → if SKIP, exit immediately (fast, no heavy installs)
2. If RUN → install ffmpeg via apt-get (or detect if already present)
3. Install Playwright Chromium if not already installed
4. Run the pipeline

Trade-offs to consider

• Requires sudo access inside the action for apt-get and
  playwright install --with-deps. Works on GitHub-hosted runners; may not
  work on all self-hosted runners.
• We should detect whether ffmpeg/Playwright are already installed and skip
  re-installing, so users who pre-install (for caching reasons) aren't penalised.
• Browser caching is currently the user's responsibility (via actions/cache).
  We could either document the cache pattern or manage it inside the action
  using @actions/tool-cache / @actions/cache.
• We may want to keep both options: a self-contained: true input (default)
  that installs deps internally, and self-contained: false for users who
  manage their own installs.

Acceptance criteria

• A minimal workflow with only uses: DeDuckProject/git-glimpse@v1 produces
  a recording on a PR with UI changes
• When the trigger check skips, the job completes in < 30 seconds (no heavy
  installs)
• Works on ubuntu-latest GitHub-hosted runners
• Documented behaviour for self-hosted runners (graceful error or skip if
  sudo unavailable)

Related

Deferred from the CI performance optimisation work (PR #TODO) that added
check-trigger as the interim user-facing solution.