feat: Add Pi agent support

[shrwnsan]

Summary

• Add Pi coding agent (pi.dev) as an opt-in tool
• Add AGENTBOX_INCLUDE_PI build arg (default: false)
• Add --tool pi CLI flag for running Pi instead of Claude
• Auto-enables Pi installation when --tool pi is selected
• Mounts ~/.pi directory for Pi configuration persistence
• Add .env.example template for API key configuration
• Update tool error messages to include 'pi' option

This PR is independent of feat/config-file (PR #57).

Changes

Pi Agent Support

• New AGENTBOX_INCLUDE_PI build arg (default: false)
• New --tool pi CLI flag for running Pi instead of Claude
• Auto-enables Pi installation when --tool pi is selected
• Mounts ~/.pi directory for Pi configuration persistence

.env.example Template

• Comprehensive template with all Anthropic environment variables
• Documents model selection variables for Claude Code
• Lists optional LLM provider API keys (OpenAI, Gemini, Mistral, Groq, etc.)

Test plan

• Build image with --tool pi and verify Pi installs correctly
• Verify agentbox --tool pi starts Pi agent
• Confirm .env.example template covers necessary API keys
• Test all three tool selections: claude, opencode, pi