Add api-designer agent - Kotlin-first API design

[Canato]

API Designer Agent

This PR adds a READ-ONLY API design agent for new public APIs.

Design Principles

• Kotlin-First: Named parameters, extension functions, data classes
• ≥2 Alternatives: Must evaluate multiple approaches with trade-offs
• Backward Compatibility: Breaking changes require migration path
• Minimal Public Surface: Internal by default
• Documentation Required: KDoc + examples mandatory

Agent Configuration

• Level: 3 (Complex analysis)
• Model: Sonnet
• disallowedTools: Write, Edit
• Role: Designs, never implements

Outputs

• Design document with rationale
• Alternative approaches evaluated
• Trade-off analysis
• Usage examples
• Backward compatibility impact

Depends On

• Add CLAUDE.md - AI Development Guide #685 (CLAUDE.md base)

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Wrote by Claude