Add optional headed browser mode

[maximus792]

Summary

• add an opt-in COMPUTER_BROWSER_MODE=headed that launches full Chromium while preserving the existing headless default
• start a private Xvfb display with collision-free -displayfd allocation and stop the computer if that display dies
• pass the mode through Compose and supervisor-managed per-Bot computers, with configuration and recreation guidance

This reuses the existing screencast and take-the-wheel flow; it does not expose X11/VNC or change the browser-control API.

Verification

• agent-computer: 215 passed, 14 integration-gated skipped; typecheck passed
• supervisor: 16 non-Docker tests passed; typecheck passed
• repository formatting and lint passed
• headed container on a Linux Docker host navigated with full Chromium and no --headless flag
• killing its owned Xvfb process made the computer close cleanly with exit code 1, ready for the container restart policy

The full supervisor Docker suite has an unrelated baseline failure in the older-image readiness fixture on current main; the changed supervisor environment tests pass.

[davidmckayv]

Deep-reviewed against live code (correctness, governance, no vendor/secret/scale issues). Composed build+tests green. CHANGELOG/format rebase on CI-validated substance.