Add AI agent definitions and skills

[brendankowitz]

Summary

• Adds four AI agent definitions under .github/agents: coding-agent, complex-coding-agent, fast-coding-agent, and well-architected-agent, each with delegation and collaboration guidance
• Adds supporting skills: create-adr, engineer-mode, and four Well-Architected review skills (full, performance, reliability, security)
• Adds AGENTS.md for AI agent onboarding context and updates CLAUDE.md and .github/copilot-instructions.md

SQL skills are tracked separately in feature/sql-agent-skills.

AB#183415

Test plan

• Agent definitions are discoverable and parseable by AI tooling
• AGENTS.md provides useful context for AI agents working in this repo

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.34%. Comparing base (1b30c40) to head (f72ec7b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5599      +/-   ##
==========================================
- Coverage   77.59%   77.34%   -0.25%     
==========================================
  Files         997      997              
  Lines       36559    36559              
  Branches     5549     5549              
==========================================
- Hits        28367    28277      -90     
- Misses       6820     6906      +86     
- Partials     1372     1376       +4     

see 13 files with indirect coverage changes

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