Implements run_tui() with RAII terminal guard (TerminalGuard), panic hook
for safe cleanup, and 50ms poll event loop. Placeholder renderer draws a
bordered box. Wires into stubs.rs replacing the coming-soon stub.
Closes claudbg-ut9q
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Defines Screen, Focus, AppState, SessionListItem with state transition
methods (new, enter_transcript, go_back). Reuses existing RawEntry and
AgentRef types. Includes 11 unit tests.
Closes claudbg-nq36
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>