Docs: Add Moxie agent guidance

[moxie-docs]

What Moxie is changing

This PR keeps repository-level agent guidance current so Codex, Cursor, Claude, and other coding agents use Moxie's live MCP context before editing.

• AGENTS.md holds a short, always-on pointer to the Moxie Docs MCP server and the moxie-docs skill.
• .claude/skills/moxie-docs/SKILL.md and .agents/skills/moxie-docs/SKILL.md add a triggered skill describing the edit-then-document workflow. It reads conventions, verified commands, and gaps live from the MCP server, so it stays correct without churn as your codebase changes.

Why this helps

• Agents get repo conventions, docs patterns, gaps, and verified commands on demand instead of re-crawling the repo.
• Feature work gets a clear docs expectation without forcing filler documentation.
• The skill is generated from your repo slug only; it re-opens a PR like this one only on a rename or a Moxie workflow update, not on every convention change.

Source paths

• moxie-docs/
• docs/