Highlight warm build servers in README

[neebs12]

What changed

• promotes warm-cache reuse into a dedicated top-level README feature
• adds an ASCII lifecycle showing create, run, suspend, resume, and cache reuse
• includes a copyable three-job GitHub Actions workflow
• explains warm-pool capacity, lease handling, benefits, and trust boundaries

Why

The previous warm-cache subsection documented the inputs but did not make the build-server lifecycle or practical value immediately visible. This gives readers a concrete path from the ordinary ephemeral workflow to opt-in warm reuse without hiding the security tradeoffs.

Validation

• git diff --check
• npm audit --audit-level=high (zero vulnerabilities)
• npm run check

CI dependency note

The first PR run exposed newly published high-severity advisories in transitive development dependencies already locked on main. The PR includes a lock-only refresh for brace-expansion, postcss, and nanoid so the required audit gate can proceed; no direct dependency or runtime code changed.