Install the chromium browser via the install-browser hook

[andrewbelcher]

Problem

The post-start hook ran playwright-cli install chromium, which isn't a valid command — install initializes a workspace and takes no positional arguments, so it errored on every start and installed no browser.

The add-on's own test suite masked this: it runs in a brand-new project, so hook 1's npm install --save-dev @playwright/cli provisions a browser as a side effect. In an already-initialized project (one already depending on @playwright/cli), the npm install/ci hooks are skipped, leaving the broken hook as the only provisioner — so no browser is installed and playwright-cli open fails with Browser "…" is not installed.

Fix

Use the correct playwright-cli install-browser chromium, and install the full build rather than --only-shell.

The full chromium download includes the headless shell, so a single install satisfies:

• headless consumers sharing this cache (Vitest/Storybook browser tests — the add-on's whole reason for existing), and
• configs that select the full Chrome-for-Testing build via channel: "chromium" (which resolves to chrome-for-testing).

--only-shell installs only the headless shell, which can't satisfy a channel: "chromium" config — so it's dropped.

Test

• Adds assert_browser_reinstalled_by_hook, run in both the directory and release pipelines: wipes the shared browser cache and restarts (npm install/ci are skipped because node_modules is intact), so the install-browser hook — not npm's install-time download — must provision the browser. This mirrors the already-initialized-project path and removes the masking that hid the bug.
• Tightens the config-print assertion to match "browserName": "chromium" specifically, instead of a bare chromium substring that also matches chromiumSandbox.

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