fix(setup): bound Playwright bootstrap and preserve skill registration

[time-attack]

On some Macs, setup could wait forever while checking or downloading Chromium. Retrying started more stuck installers, and setup stopped before registering commands such as /qa and /ship.

Setup now limits how long browser checks and downloads may run, allows only one setup at a time, cleans up timed-out child processes, and still registers commands when Chromium is unavailable.

Technical changes

• use Node instead of Bun for Chromium launch probing on macOS
• bound launch probes and Chromium installation
• terminate timed-out descendant processes
• serialize setup with a portable stale-PID lock
• continue skill registration after Chromium failure
• add focused regression coverage

Live verification

Identical baseline/candidate probes were run in isolated clones and HOME directories:

• TideDra/zotero-arxiv-daily: fail → pass
• loperanger7/gstack-auto: fail → pass
• garrytan/gbrain: fail → pass
• browserbase/stagehand normal-install control: pass → pass
• makeplane/plane normal-install control: pass → pass

The evidence gate passed and no marked child processes remained.

Secondary tests

• bash -n setup
• bun test test/setup-playwright-deadline.test.ts test/setup-windows-fallback.test.ts — 12 passed

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