feat(cli): add --debug flag to write all trace logs to ./debug.txt

[echobt]

Summary

This PR adds a --debug flag that enables comprehensive debug logging to help troubleshoot issues when the agent doesn't return expected output.

Changes

New --debug flag

When --debug is passed:

• All trace-level logs are written to ./debug.txt in the current working directory
• Logs include:
  • Module paths (target)
  • Thread IDs and names
  • Source file paths and line numbers
  • Timestamps
• Non-blocking writer is used for performance
• A confirmation message is printed to stderr showing the debug file location

Usage

# Start TUI with debug logging
cortex --debug

# Run a command with debug logging  
cortex --debug run "explain this code"

# Exec mode with debug logging
cortex --debug exec "run tests"

The generated debug.txt file will contain all Rust tracing events and internal diagnostics, which is invaluable for troubleshooting when the agent doesn't produce expected output.

Implementation Details

• Uses tracing-appender for non-blocking file writing
• Sets log level to trace to capture everything
• Guard pattern ensures logs are properly flushed when the program exits
• Works in both TUI and non-TUI modes