docs: add skill guide for building with hypercerts lexicons

Without this patch, AI agents building downstream applications with
the @hypercerts-org/lexicon package lacked a comprehensive guide on how
to consume the package effectively. The README.md and AGENTS.md files
focus on repository development rather than downstream usage.

This is a problem because agents need clear guidance on:
- Which published releases to use (npm package vs git tags)
- How to import types, NSIDs, and validation functions
- Common patterns for creating activities, collections, attachments
- The relationship map between different lexicon types
- How to use strongRefs and proper record keys

This patch solves the problem by creating a new skill
.agents/skills/building-with-hypercerts-lexicons/SKILL.md that provides:
- Quick start examples with the AT Protocol agent
- Full reference tables for all lexicons and NSIDs
- Code examples for common patterns (activities, collections, etc.)
- Package entry point documentation
- Validation patterns (generic and type-safe)
- Visual relationship map showing how records link together

The AGENTS.md file is updated to point downstream developers to this
skill guide instead of the README, ensuring the right audience gets the
right documentation.

Co-authored-by: Claude (Sonnet 4.5) <claude-sonnet-4-5@noreply.anthropic.com>

Author: aspiers