Add a `/cwd <path>` command that updates the active TUI session to a
new working directory without forcing a restart. The app rebuilds
cwd-sensitive config, refreshes local state, and keeps the session
thread alive.
Refresh core session configuration on cwd changes so project
instructions and turn context stay aligned with the new directory.
This keeps AGENTS.md updates model-visible while avoiding unrelated
sandbox-path changes in the same review.