feat(screenshot-change): add shoot runner and fix stale verbs

[moritzwilksch]

Makes the screenshot skill cheap for agents to use. It also fixes the per-file verbs, which had timed out on main.

Runner

A scenario is a small module in the agent's scratch directory:

export const repo = { before: { 'a.py': 'x = 1\n' }, after: { 'a.py': 'x = 2\n' } };
export default async ({ page, shot, selectLines }) => {
  await selectLines('a.py', 1);
  await page.keyboard.type('Wait...');
  await shot('composer', page.locator('textarea').first());
};

node .agents/skills/screenshot-change/scripts/shoot.mjs scenario.mjs --before origin/main runs it and prints only this:

/tmp/…/lig
  ellipsis-after.png 752×128
  ellipsis-before.png 752×128
  ellipsis-compare.png 788×107  ← read this to verify

• Demo repos: demoRepo creates them in the temp dir with an explicit -C and inline identity, so nothing can touch the checkout or its config.
• Before state: --before runs a git archive copy of the base revision with its own server and client. It's cached per commit, so re-runs take about 4 s, and it never swaps dist/client or registers a worktree.
• Quiet builds: build and ffmpeg output is captured and shown only on failure. It used to be about 370 lines per build, and a before/after run built three times. The client rebuilds only when its sources change.
• One read to verify: one labelled compare image per shot.
• Short failures: the error, the scenario line, and a 1x failure-<side>.png; exit status 1.
• Self-test: scripts/selftest.mjs exercises every verb, and lookup errors name the likely cause.
• Docs: SKILL.md is about a third smaller and is organised around the runner, with a verb table that matches the harness.

💥 Breaking: withBaseClient, installClient, CLIENT_DIR and scenario.template.mjs are removed. Write scenarios for shoot.mjs, or call baseCheckout(rev) and startDiffle({ root }) directly.

Stale verbs (first commit)

header, viewed, collapsed, setViewed, toggleCollapse and selectLines all timed out on main:

• Header lookup: fileItem looked for the viewer's default [data-title] header, which diffle has replaced with its own slotted FileHeader. It now matches the path span's title there.
• Tooltips: TooltipHost removes title from the hovered control, so the verbs now move the pointer to the top-left corner first.
• Cursor line: pressing the number of the cursor's selected line toggled the selection off, so selectLines presses Escape first. fix(review): comment on the cursor's line when its number is pressed #239 fixes the app side.

Verified

• selftest.mjs: all verbs pass, including a comment stored on a.py line 2.
• A before/after run against c9dc8d2506 shows the ligature bug fixed by fix(client): disable font ligatures #237 (image above).
• A video scenario and a failing scenario behave as described.
• Also tested: an existing repo path with revs, setup seeding threads, and an uncommitted demo repo.
• Lint, format and npm test pass.

[moritzwilksch]

Superseded by #242, which ports this onto the e2e harness from #214.