doc: add portable agentic instructions

[fredbi]

Summary

• Add AGENTS.md (symlink) for cross-tool AI agent support
• Add contributions.md and github-workflows-conventions.md rules
• Update go-conventions.md and copilot-instructions.md

Test plan

• Verify symlinks resolve correctly
• Confirm no functional code changes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.78%. Comparing base (93dabab) to head (f6a7807).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #131   +/-   ##
=======================================
  Coverage   80.78%   80.78%           
=======================================
  Files           4        4           
  Lines         229      229           
=======================================
  Hits          185      185           
  Misses         28       28           
  Partials       16       16           

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