chore: make fresh worktrees run e2e out of the box

[tomasz-tomczyk]

Summary

A freshly created wt worktree of crit-web couldn't run the Playwright e2e suite without manual setup:

• The [post-start] copy = "wt step copy-ignored" hook only copies git-ignored files that exist in the source checkout. The canonical crit-web/ checkout has no root node_modules — the root package.json Playwright deps (@playwright/test, @axe-core/playwright, @types/node) were never installed there, so there was nothing to copy. A fresh worktree hit Cannot find module '@playwright/test'.
• crit-web had no local e2e runner. CI (.github/workflows/e2e.yml) installs deps inline, but a developer got nothing — manual npm install + asset build was required first.

This adds a single documented command, mise run e2e, that is idempotent and self-bootstrapping, mirroring .github/workflows/e2e.yml:

• npm ci (root Playwright deps — the robust fix, since copy-ignored can only copy what already exists in the source)
• npx playwright install chromium
• npm install --prefix assets
• MIX_ENV=test E2E=true mix deps.compile then mix compile (deps.compile guards against a stale copied _build/test — a fresh worktree's copied build can be missing a dep's Erlang parser .beam; CI never hits this since it builds the test tree fresh from cache)
• mix assets.build
• depends = ["db:start"] ensures the Postgres container is up (DB_PORT 5433 host → 5432 container, via mise.toml)
• npx playwright test — extra args forward through (e.g. mise run e2e e2e/accessibility.spec.ts)

playwright.config.ts's managed webServer starts Phoenix and runs ecto.create/migrate itself.

Mirrors the intent of tomasz-tomczyk/crit#621 ("chore: make fresh worktrees and e2e work out of the box") for crit-web — an install step in the runner is the robust fix because copy-ignored can only copy what already exists in the source checkout.

Review

• Code review: N/A (config + docs only; no app/test/CSS code)
• Parity audit: N/A

Test plan

• Simulated a fresh worktree (no root node_modules) and ran mise run e2e e2e/accessibility.spec.ts. The task bootstrapped everything (npm ci, playwright install, asset deps, test-env compile, assets.build, DB), started Phoenix, and ran the suite with no manual npm install. 2 specs passed; the 1 dark-theme color-contrast failure is pre-existing on main (subject of a separate task) and unrelated to this diff (which touches only mise.toml + AGENTS.md).
• Confirmed the usage field replaces the deprecated arg() template function (no mise deprecation warning) and arg passthrough to a spec path works.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.42%. Comparing base (10b2fd7) to head (9b35ddf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #243   +/-   ##
=======================================
  Coverage   82.42%   82.42%           
=======================================
  Files          89       89           
  Lines        3089     3089           
=======================================
  Hits         2546     2546           
  Misses        543      543           

Flag	Coverage Δ
unit	82.42% <ø> (ø)

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