docs: update CLAUDE.md with language spec structure guidance

[xgopilot]

Requested by @xushiwei

This PR updates CLAUDE.md to incorporate XGo's language specification structure information as requested in issue #2504.

Summary

• Added comprehensive documentation about XGo's dual-level specification system
• Documented MiniSpec (recommended best practices for all users) and FullSpec (complete syntax for experts)
• Included clear guidance on which files to update for each spec type
• Added decision criteria for classifying new syntax features
• Provided update process for maintaining specifications

Changes

• Language Specification Structure section added to CLAUDE.md with:
  • MiniSpec documentation: purpose, audience, and files (doc/spec-mini.md, doc/spec/mini/mini.xgo)
  • FullSpec documentation: purpose, audience, and files (doc/spec.md)
  • Decision criteria for determining whether new syntax belongs in MiniSpec or FullSpec
  • Step-by-step update process for documentation

Context

Closes #2504

This enhancement ensures that when adding new syntax to XGo, developers have clear guidance on:

1. The distinction between MiniSpec and FullSpec
2. Which specification documents need to be updated
3. How to determine the appropriate classification for new syntax features

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.98%. Comparing base (db25915) to head (38fb947).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2505   +/-   ##
=======================================
  Coverage   93.98%   93.98%           
=======================================
  Files          34       34           
  Lines        9613     9613           
=======================================
  Hits         9035     9035           
  Misses        413      413           
  Partials      165      165           

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