Integrate voice input into the TUI and add turn cancellation

[alexkroman]

Summary

This PR integrates voice input/output into the code agent TUI, allowing users to speak their requests and hear summaries of replies read back. It also adds turn cancellation (Escape) and a safer quit mechanism (Ctrl-C double-press confirmation).

Key Changes

Voice Integration in TUI

• Added _VoiceIO protocol to abstract voice session interface (listen/speak)
• TUI now accepts optional voice parameter and routes spoken turns into the prompt
• Implemented _capture_voice_turn() to listen for spoken input on a background thread
• Implemented _voice_followup() to read back a spoken summary after each reply
• Voice degrades gracefully to typed input if microphone is unavailable (sets _voice_typed flag)

Turn Cancellation & Safer Quit

• Added action_interrupt() (Escape key) to cancel a running agent turn without quitting
• Added action_quit_or_interrupt() (Ctrl-C) that interrupts running turns or requires double-press to quit when idle
• Implemented _turn_running() to check if prompt is disabled (turn in flight)
• Implemented _cancel_turn() to set the session's cancel flag cooperatively
• Added _quit_pending flag and _arm_quit_pending() to show "Press Ctrl-C again to quit" hint

Session Streaming & Cancellation

• Added _SupportsStream protocol to detect agents that support incremental streaming
• Refactored CodeSession.send() to call new _run() method that streams events incrementally
• _run() checks _cancel flag between steps, allowing cooperative turn interruption
• _resolve_interrupts() also respects the cancel flag to stop approval loops early
• Added request_cancel() method to set the cancel flag from another thread

Voice Readback Improvements

• Added spoken_summary() function to strip code (fenced and inline) from replies before TTS
• Caps spoken text to 600 characters to keep readback brief
• Returns generic fallback message when reply is all code
• Updated voice REPL sink to use spoken_summary() instead of reading full text

UI Polish

• Changed background color from #0b0e16 to pure black (#000000) for cleaner appearance
• Updated key bindings: Escape for interrupt, Ctrl-C for interrupt/quit, Ctrl-Q for unconditional quit

Testing

• Added comprehensive test suite in test_code_tui_voice.py covering voice integration scenarios
• Added tests for turn cancellation and quit mechanics in test_code_tui.py
• Added streaming agent test in test_code_agent.py to verify incremental event emission
• Added spoken_summary() tests in test_code_voice.py
• Updated command dispatch tests to verify voice session is passed to TUI

Implementation Details

• Voice operations (listen/speak) run on daemon threads to avoid blocking the UI
• call_from_thread() is used to safely update UI state from voice worker threads
• The cancel flag is a threading.Event checked at step boundaries, enabling cooperative cancellation without killing threads
• Streaming agents (real compiled graphs) emit events incrementally; test doubles that only implement invoke() emit once at the end
• The quit hint timer automatically clears the _quit_pending flag after 3 seconds

https://claude.ai/code/session_01FUL1Y7QWgAUDTRdQtK2qCJ