chore: add xgopilot config to use claude-4.5-opus model

[xgopilot]

Requested by @luoliwoshang

Fixes #71

This PR adds xgopilot configuration to use the claude-4.5-opus model, similar to the configuration added to:

• goplus/llgo (PR #1556)
• goplus/llcppg (PR #621)

Summary

• Added .github/xgopilot.yml with claude-4.5-opus model configuration

Changes

• Created .github/xgopilot.yml file with the following configuration:

  claude:
    model: "claude-4.5-opus"

Benefits

• Ensures consistent AI assistant model usage across goplus repositories
• Enables use of the latest Claude Opus 4.5 model for better code assistance

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.05%. Comparing base (525e603) to head (f981be0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #72   +/-   ##
=======================================
  Coverage   47.05%   47.05%           
=======================================
  Files           4        4           
  Lines          34       34           
=======================================
  Hits           16       16           
  Misses         18       18           

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