feat(screenshot): add a capture runner and fix e2e verb papercuts

[moritzwilksch]

Makes screenshotting a change cheap for agents, on top of the e2e harness from #214, and fixes two papercuts users reported. Replaces #238, which targeted the harness #214 removed.

Runner

A scenario exports one interaction function and, optionally, a repo. The runner owns the server, browser, before/after runs and cleanup.

import type { ScenarioContext } from '<checkout>/scripts/screenshot.ts';
export default async ({ page, shot, selectLines }: ScenarioContext) => {
  await selectLines('tally/ledger.py', 3, 3);
  await page.keyboard.type('Wait...');
  await shot('ellipsis', page.getByPlaceholder('Leave a comment…'));
};

npm run screenshot -- lig.ts --before c9dc8d2506 prints only this:

/tmp/…/lig
  ellipsis-after.png 726×128
  ellipsis-before.png 722×128
  ellipsis-compare.png 760×107  ← read this to verify

• Repo: the fixture repository by default. A scenario can export inline { before, after } files (demoRepo: temp dir, git -C, inline identity) or a repo path.
• Before state: --before runs a git archive build of that revision with its own server and client, through startDiffle({ root }). It is cached per commit, and never swaps dist/client or registers a worktree.
• Quiet builds: buildClient prints only on failure and skips a build whose sources haven't changed, which also speeds up the e2e global setup. ffmpeg is quiet and the intermediate webm is removed.
• Compare image: one labelled image per shot, so one read verifies a change.
• Failures: the error, the scenario line, and a 1x failure-<side>.png; exit status 1.
• Docs: SKILL.md and SETUP.md are rewritten around the runner.

-export async function withBaseClient<T>(rev: string, fn: () => Promise<T>): Promise<T>
+export function baseCheckout(rev: string): string
 export interface DiffleOptions {
+  root?: string; // the diffle checkout to run, server and client
 }

💥 Breaking: withBaseClient, installClient, CLIENT_DIR and scenario.template.mts are removed. npm run screenshot now takes a scenario for the runner.

Harness fixes (test/e2e/browser.ts)

• Off-screen files: the viewer renders only files near the viewport, so helpers acting on a listed but off-screen file timed out (reported). Header helpers now walk there with J/K, keeping a collapsed file collapsed; line helpers use gotoFile, which also expands it.
• Mid-scroll press: line helpers wait for the target row to stop moving before pressing. Before, a press landed during the scroll correction and opened nothing.
• Clear failures: a file with no lines (binary, empty, rename-only, mode-only) fails in about 2 s with a message saying so, not a 15 s timeout. A missing line lists the rendered lines. selectLines waits for the composer and says if it never opens.
• Tooltip: toggleCollapse moves the pointer away first. The app's tooltip removes title from the hovered button, so a second toggle found nothing.
• Older revisions: fileItem also accepts headers from before data-path, so --before works on older revisions.
• Thread shape: SKILL.md documents it: anchor.path, messages[0].body, no comments (reported).

test/e2e/harness.spec.ts pins every helper in CI, so markup changes break there instead of in an agent's script.