Make chat widget harness git setup opt-in

[cbusillo]

Summary

• Keeps the default ChatWidgetHarness isolated and PATH-agnostic.
• Adds ChatWidgetHarness::new_in_git_repo() for tests that explicitly need git semantics.
• Updates the auto-drive write-agent test to opt into a git-backed harness.

Validation

• cargo test --manifest-path code-rs/Cargo.toml -p code-tui chatwidget::tests::auto_handle_decision_launches_cli_agents_and_review --features test-helpers -- --nocapture
• cargo test --manifest-path code-rs/Cargo.toml -p code-tui chatwidget::tests::missing_agent_clis_start_disabled_in_overview --features test-helpers -- --nocapture
• ./build-fast.sh