Add Claude Code agents and CLAUDE.md

[TsekNet]

Summary

• Add CLAUDE.md with project conventions, referencing existing docs
• Add 2 specialized agents:
  • diff-reviewer: semantic diff engine, match keys, parser safety, merge correctness
  • output-reviewer: terminal/JSON/markdown renderer correctness and consistency

Design decisions

• Agents reference docs/Architecture.md instead of duplicating content
• All agents use model: opus
• No platform-reviewer (fleet-plan is pure Go, no OS-specific code)
• No security-auditor (read-only GET client, minimal attack surface)

Test plan

• Verify agents are visible via /agents in Claude Code
• Run diff-reviewer against a diff engine change

Ref #34