chore: add agent instruction files as symlinks to .rules.md

[vdusek]

Summary

• Add .rules.md file containing project conventions and coding guidelines
• Create AGENTS.md, CLAUDE.md, and GEMINI.md as symlinks pointing to .rules.md
• Update .gitignore accordingly

Test plan

• Verify symlinks resolve correctly
• Lint and type checks pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.50%. Comparing base (69d02e9) to head (0d27884).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #651      +/-   ##
==========================================
- Coverage   96.55%   96.50%   -0.05%     
==========================================
  Files          46       46              
  Lines        4383     4383              
==========================================
- Hits         4232     4230       -2     
- Misses        151      153       +2     

Flag	Coverage Δ
integration	94.68% <ø> (+0.06%)	⬆️
unit	77.20% <ø> (ø)

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