Add Kernel-backed Harbor control arm

[rgarcia]

Summary

• add native Kernel browser runtime support with replay capture and reliable cleanup
• create headful, stealth Kernel browsers without overriding the provider's default viewport
• add a Kernel-backed mode to clawbench-harbor-adapt that attaches ClawBench instrumentation to one pre-created browser
• configure stock Harbor agents with pinned Playwright MCP through the credential-free CDP bridge
• stop stock Claude Code and Codex cleanly when ClawBench records the final request
• intercept the Red Cross ZIP lookup request rather than blocking the chapter-finder landing page
• emit Harbor 0.21-compatible numeric verifier metrics and timestamped interception evidence

Validation

• uv run pytest -q (203 passed)
• uv run ruff check src/clawbench tests
• uv run ruff format --check src/clawbench tests
• uv run --frozen pyright src/clawbench tests
• Playwright MCP control on stealth Kernel browsers: 3/3 intercepted=1, judge_match=1, reward=1
• median agent-start-to-interception: 40.8s (32.6–68.6s)
• median first-browser-tool-to-interception: 31.1s (24.5–47.3s)
• median stop propagation: 47ms (13–73ms)
• all replay recordings retained and all browser deletions verified