Summary:
- Store a lazily-created sandboxed terminal temp directory on each agent
thread.
- Persist the tempdir path and remove it when deleting archived threads.
- Update sandbox prompt wording to describe per-thread tempdir reuse.

Tests:
- cargo fmt --package agent --package agent_ui --package zed
- cargo test -p agent db::tests::test_sandboxed_terminal_temp_dir --
--nocapture
- cargo test -p agent
db::tests::test_delete_thread_removes_sandboxed_terminal_temp_dir --
--nocapture
- cargo check -p agent_ui -p zed

Release Notes:

- Improved agent terminal sandboxing to preserve temporary files across
commands in the same thread

---------

Co-authored-by: MartinYe1234 <52641447+MartinYe1234@users.noreply.github.com>
Co-authored-by: Martin Ye <martin@zed.dev>