Document context budget and file size guidelines for contributors

[glacierphonk]

Summary

• Adds "Context budget and file sizes" section to CONTRIBUTING.md
• Explains the loading tier model (always / on invoke / on demand / never auto-loaded) with context cost estimates
• Provides typical session costs for simple vs. full naming processes
• Includes contributor guidelines on file size limits and when to split files
• Complements the existing user-facing "How the skill uses context" section in README.md

Closes #137