Harden reliability, security, and UX across core modules

- Add thread safety: _anthropic_history_lock and _send_lock in ai_client to prevent concurrent corruption
  - Add _send_thread_lock in gui_2 for atomic check-and-start of send thread
  - Add atexit fallback in session_logger to flush log files on abnormal exit
  - Fix file descriptor leaks: use context managers for urlopen in mcp_client
  - Cap unbounded tool output growth at 500KB per send() call (both Gemini and Anthropic)
  - Harden path traversal: resolve(strict=True) with fallback in mcp_client allowlist checks
  - Add SLOP_CREDENTIALS env var override for credentials.toml with helpful error message
  - Fix Gemini token heuristic: use _CHARS_PER_TOKEN (3.5) instead of hardcoded // 4
  - Add keyboard shortcuts: Ctrl+Enter to send, Ctrl+L to clear message input
  - Add auto-save: flush project and config to disk every 60 seconds

session_logger.py

@@ -26,6 +26,7 @@ scripts/generated/
 Where <ts> = YYYYMMDD_HHMMSS of when this session was started.
 """
 
+import atexit
 import datetime
 import json
 import threading
@@ -71,6 +72,8 @@ def open_session():
     _tool_fh.write(f"# Tool-call log — session {_ts}\n\n")
     _tool_fh.flush()
 
+    atexit.register(close_session)
+
 
 def close_session():
     """Flush and close both log files.  Called on clean exit (optional)."""