Improve docs for LLM tools

[pmillspaugh]

• Ship v1 prompting docs (https://docs.val.town/guides/prompting)
• Store system prompt somewhere other than Townie?
• Custom prompts, e.g. for Claude Code: auto-sync'ing edits, limitation that Claude can't create triggers / instruct the user to do that manually, provide the user with the URL, etc.