Build 78

feat(browser): auto-install playwriter via npm/bun/pnpm on first use

Previously, if playwriter wasn't on PATH, every exec:browser call
failed silently with '20 retries on port 9222' — Chrome launched fine
but the CDP session-creation command targeting playwriter never found
the binary and spawned nothing.

Now ensure_playwriter() runs once per runner lifetime: if
which::which("playwriter") fails, it attempts a global install via
the first available package manager (npm, bun, pnpm) and retries the
lookup. If install fails, falls back to the old literal-string
behaviour so error surface is unchanged for environments with no
package manager.