Add a script that screenshots a PR before and after

[crimsonsunset]

Summary

scripts/pr_screenshots.py captures before/after screenshots of a PR (merge-base vs head) at desktop and phone width in light and dark, and with --post writes them into the PR description. It exists so UI PRs can carry visual proof without taking shots by hand across branches.

Changes overview

• scripts/pr_screenshots.py: serves each commit from a throwaway git worktree with its own SQLite database and a private Redis (port 6391), drives headless Chromium, uploads images to the fork and rewrites a marked block of the PR description.
• scripts/pr_screenshots_seed.py: seeds a shots user, movies, and a band/album/tracks. Tolerates fields and routes a commit does not have yet.
• docs/agents/pr_screenshots.md: usage, page selection, requirements, limits.
• AGENTS.md: one line in the Agent Docs list.

Key technical decisions

• Worktrees, not branch switching, so the working tree is never touched.
• Images upload through the undocumented uploads.github.com/user-attachments/assets endpoint using the gh token. It only accepts repos you can push to, so images go to the fork and are referenced from the upstream PR.
• Pages per PR live in a PR_PAGES dict, not a config file.

Test plan

• Ran on Add per-media-type tile settings #1370, Add a music track detail page #1372, Make Home Screen settings usable #1373, Show SoundCloud and Spotify play links on a music album #1374, Let a Recently Played row show album, artist, track, or all three #1375 and Show genres on detail pages and fill them on scrobble #1311 end to end with --post; uploaded images load
• uv run --no-sync python scripts/pr_screenshots.py <pr> --no-before on a PR of your own
• uv run --no-sync ruff check scripts/pr_screenshots.py scripts/pr_screenshots_seed.py

Docs

• docs/agents/pr_screenshots.md