feat: add canopy_docs CLI for LLM documentation generation

[tylerclair]

Adds canopy/scripts/canopy_docs.py as a new entry point registered in
pyproject.toml. Provides three subcommands:

• generate-llms: writes a static llms.txt describing CanvasSession,
  CanvasClient, CanvasAPIError, Canopy kwargs, pagination, async usage,
  and common usage patterns
• generate-index: parses generated API files via AST and emits a compact