docs: add pitfalls section and grant Edit tool access

[dvhthomas]

Summary

• Adds a "Common Pitfalls" section to CALCMARK-AGENT.md covering immutable variables, unit propagation, NL-first guidance, and stderr error handling
• Adds a condensed "Common Pitfalls" subsection to the Claude Code SKILL.md
• Grants Edit tool access in SKILL.md frontmatter so agents can incrementally fix .cm files instead of rewriting them entirely

Context

These changes are based on real agent usage where:

1. The agent used raw arithmetic (customers / 10) instead of NL capacity form, producing wrong units
2. The agent reassigned an immutable variable and needed to Edit the .cm file to fix it — but lacked Edit permission and had to rewrite the whole file
3. The agent piped stderr errors into a JSON parser, causing a silent failure

Companion PR in go-calcmark adds the same pitfalls to the full agent-integration docs.

Test plan

• Verify allowed-tools in SKILL.md now includes Edit
• Confirm pitfalls sections are correctly placed before Security/Error Handling sections

🤖 Generated with Claude Code