Add editor command (and make it the default when no args provided)

[oeo]

Adds an interactive editor mode to the LLM CLI:

I noticed llm was just hanging without any args and had a personal tool that allowed me to open my system default editor and sort of draft a more complex prompt and thought the feature was useful.

• New llm editor command to compose prompts in a system editor
• Default behavior now opens an editor when no arguments are provided
• Supports specifying custom editor with -e/--editor flag

Usage

# Open default editor
llm
llm editor

# Open with specific editor
llm editor -e vim