doc: add portable agentic instructions

[fredbi]

Summary

• Add AGENTS.md (symlink) for cross-tool AI agent support
• Add contributions.md rule (DCO, linting, mandatory tests, problem statement)
• Add github-workflows-conventions.md rule
• Update go-conventions.md to match canonical version

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 82.53%. Comparing base (97319b8) to head (607947d).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #114   +/-   ##
=======================================
  Coverage   82.53%   82.53%           
=======================================
  Files           2        2           
  Lines         292      292           
=======================================
  Hits          241      241           
  Misses         35       35           
  Partials       16       16           

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