Add Gemini CLI support for AI analysis

[biwakonbu]

Summary

Extend AI provider support to include Google's Gemini CLI alongside the existing Claude Code CLI integration.

Current State

• Currently only supports Claude Code CLI for AI analysis
• Uses hardcoded Claude Code CLI commands in internal/ai/ package

Proposed Implementation

1. Add Gemini CLI Integration

   • Create gemini provider option in configuration
   • Implement Gemini-specific prompt formatting and response parsing
   • Add Gemini CLI command execution wrapper

2. Refactor AI Provider Architecture

   • Extract common AI provider interface
   • Support multiple providers with fallback mechanism
   • Allow user configuration of preferred AI provider

3. Configuration Changes

   {
     "ai_provider": "claude < /dev/null | gemini",
     "claude_settings": { ... },
     "gemini_settings": { ... }
   }

Acceptance Criteria

• Users can configure ai_provider: "gemini" in config
• Task generation works with Gemini CLI
• Existing Claude Code CLI functionality preserved
• Documentation updated with Gemini setup instructions
• Tests cover both AI providers

Implementation Notes

• Ensure prompt compatibility between providers
• Handle provider-specific response formats
• Maintain consistent task generation quality

Priority: Medium