fix(core): require visible worktree choice

[shiny-code-bot]

Summary

• strengthen the model-facing concurrent-checkout notice so the agent must visibly choose an isolated worktree or explain why it is staying before edits
• add a request-capture regression test proving the notice is sent as a developer message before the user task

Validation

• cargo test -p code-core --test active_session_warnings concurrent_checkout_notice_in_model_request_requires_visible_worktree_choice
• cargo test -p code-core --test active_session_warnings
• ./build-fast.sh

Follow-up

• Open a follow-up issue to prove the behavior in the exec/TUI harness and decide whether stronger product-level enforcement is warranted.