feat: add --one-shot flag for sessionless single-prompt execution

[12310312long]

Summary

Add a --one-shot flag that allows running a single prompt without initializing a git repository, then exits immediately after delivering the response. This is useful for quick one-off tasks like explaining code or simple refactoring where you don't need a full session.

Usage

# Message via --message flag
aider --one-shot --message "explain this function" src/utils.js

# Message as positional argument (more concise)
aider --one-shot "explain this function" src/utils.js

# Message from file
aider --one-shot --message-file prompt.txt src/utils.js

Implementation

• aider/args.py: Added --one-shot argument to the "Modes" group
• aider/main.py: When --one-shot is set:
  • Forces --no-git to skip all git repository initialization
  • If no --message or --message-file is provided, treats the first positional argument as the message
  • Shows an error if no message source is found
• The existing --message handler in main.py runs the coder with the message and exits, which works unchanged

Testing

• Manually tested with both --message and positional-arg message syntax
• Verified git repo is skipped (Git repo: none)
• Verified file modifications work correctly
• All existing pytest tests pass (the one failure on Windows is pre-existing and unrelated)

Closes #5143