Add Code Bridge live browser witness

[shiny-code-bot]

Summary

• add an ignored Code Bridge live browser witness integration test
• serve a tiny loopback HTML fixture that connects as a browser app, publishes pageview/console events, reads bridge events with fetch-based SSE, and responds to screenshot/control requests
• add an explicit local contract flag for running the live witness when a real browser is available

This is the first real browser-page witness for #36. It remains opt-in/manual because it waits for a browser to open the printed local URL; normal CI still runs the fast protocol fixture.

Refs #36

Validation

• python3 -m py_compile scripts/local/e2e_agent_session_contract.py
• npx prettier --check codex-rs/code-bridge-client/tests/fixtures/live_browser_witness.html
• cargo fmt --manifest-path codex-rs/Cargo.toml --package codex-code-bridge-client -- --check
• cargo test --manifest-path codex-rs/Cargo.toml -p codex-code-bridge-client --no-fail-fast
• cargo test --manifest-path codex-rs/Cargo.toml -p codex-code-bridge-client --test live_browser_witness -- --ignored --nocapture, with the printed URL opened in the harness browser; passed twice after rendering the nonblank canvas
• Review agents: GPT review found no issues; Claude risk review found no serious issues and the noted fixture request-read brittleness was tightened before commit