Add Claude Code agents and CLAUDE.md

[TsekNet]

Summary

• Add CLAUDE.md with project conventions, build commands, proto/gRPC rules
• Add 4 specialized agents in .claude/agents/:
  • platform-reviewer: cross-platform parity (Windows/macOS/Linux)
  • proto-guardian: gRPC backward compatibility
  • security-auditor: command injection, auth, state machine review
  • integration-tester: runs testdata fixtures against local binary

Design decisions

• Agents reference docs/ instead of duplicating content
• All agents use model: opus
• No agent memory (reviewers are stateless, no cross-session context needed)
• No implementation agent (default session + CLAUDE.md + docs is sufficient)

Test plan

• Verify agents are visible via /agents in Claude Code
• Run platform-reviewer against a platform-specific diff
• Run integration-tester to validate testdata fixtures

Ref #4