docs: expand CLAUDE.md with circuit design API guidance

[lachlanfysh]

Summary

• Adds KiCad environment setup (KICAD9_SYMBOL_DIR, KICAD9_FOOTPRINT_DIR) — the most common stumbling block for new users and AI agents
• Documents core API patterns: Part, Net, Bus, Circuit, @Subcircuit with working examples
• Covers pin connection syntax (+=), hierarchy with @Subcircuit, and Bus usage
• Lists common pitfalls: missing footprints, pin numbering as strings, library name matching, NC pins
• Expands the module map with file-level descriptions (part.py, net.py, circuit.py, node.py, etc.)
• Documents the UUID scheme for schematic ↔ PCB cross-reference

Motivation

I've been using AI coding assistants (Claude Code, Codex) to design circuits with SKiDL. The existing CLAUDE.md covers development setup well but doesn't explain how to actually use SKiDL — the API, conventions, and gotchas. AI agents consistently hit the same walls:

• Not knowing to set KICAD9_SYMBOL_DIR
• Getting library names wrong ("device" vs "Device")
• Missing footprints on parts
• Not understanding @Subcircuit for hierarchy

This PR addresses all of those. No code changes — documentation only.

Test plan

• Verify CLAUDE.md renders correctly on GitHub
• No code changes, existing tests unaffected

🤖 Generated with Claude Code