fix(tui): clear resume config for new chat

[shiny-code-bot]

Summary

• Clear one-shot resume configuration before constructing fresh chat widgets for /new and ClearUi
• Clear the same resume path after onboarding completes while preserving onboarding-updated auth/trust config
• Add a regression test that exercises the /new rebuild path and verifies the new widget starts without experimental_resume

Fixes #252

Verification

• cargo test -p code-tui fresh_chat_config_clears_resume_path --features test-helpers
• ./build-fast.sh