fix(agent): start and clean up the CDP browser host on Windows

[luminary19]

Summary

The browser host never came up on Windows: port enumeration used POSIX
tooling, the DevTools ws endpoint was read only from Chrome's stderr (empty
when Windows de-elevates the browser out of an elevated parent), and orphaned
profile browsers from crashed hosts blocked relaunch (pkill cleanup is
POSIX-only). Adds netstat -ano enumeration, falls back to the profile's
DevToolsActivePort file for the endpoint, and sweeps/revalidates orphaned
profile browsers before launch. Fixes #198.

Validation

npm --prefix frontend run typecheck clean (covers the agent-runtime sources);
check:quality runs as the pre-push gate. The Windows behavior — netstat -ano
port enumeration, DevToolsActivePort endpoint fallback, orphan-profile sweep —
was verified live on Windows 11 (embedded browser navigates/snapshots from
/agent; killing the host mid-session and relaunching recovers). Linux/macOS is
unchanged: the stderr endpoint path runs first and short-circuits, and cleanup
keeps its existing behavior.

UI changes

None.

Risks / rollout notes

• The orphan sweep only targets processes whose command line references the
  dedicated local-studio-browser-profile dir — it cannot touch user
  browsers.