feat: add OpenCode integration for gstack skills

[rapidstartup]

This PR adds OpenCode integration to gstack, allowing gstack skills to be used in OpenCode via the command dialog (Ctrl+K).

What This Adds

1. OpenCode Adapter for Paperclip (packages/adapters/opencode/)
   • Server-side execution logic for running OpenCode in non-interactive mode
   • Environment validation and configuration handling
   • Output parsing and session management
   • UI configuration components for agent setup
   • CLI output formatting for terminal monitoring
2. OpenCode Command Files (opencode/commands/gstack/)
   • Makes all gstack skills discoverable in OpenCode's command dialog
   • Includes commands for: office-hours, plan-ceo-review, plan-eng-review, design-consultation, review, debug, ship, qa
   • Each command file follows OpenCode's custom command format with descriptions, allowed tools, and execution context
3. Global Installation Command (bin/gstack-opencode-setup)
   • Allows installing gstack OpenCode integration globally with gstack-opencode-setup
   • Copies command files to ~/.config/opencode/commands/gstack/ (or platform equivalent)
   • Makes gstack skills available in ALL OpenCode instances
4. Integration with Existing GSD Commands
   • Leverages the existing get-shit-done commands that already work in OpenCode
   • Provides unified access to both gstack and gsd functionality

Why This Integration Was Created

This OpenCode integration enables using gstack skills in Paperclip and Vibedash.app for better ideation workflows. It allows:

• Using gstack's structured skills (office-hours, plan-ceo-review, etc.) within OpenCode sessions
• Accessing these skills via OpenCode's command dialog (Ctrl+K) instead of Claude Code's slash commands
• Combining gstack ideation with OpenCode's multiple AI provider support and TUI features
• Creating seamless workflows between ideation (gstack skills) and implementation (OpenCode coding assistance)
• Allows Vibedash users to get the office-hours and eng and design plans into their ideas before pushing to their Build Agents

How It Works After Installation

1. Install gstack (standard process)
2. Run gstack-opencode-setup to make commands globally available (optional)
3. Open OpenCode
4. Press Ctrl+K to open the command dialog
5. Type gstack: to see all available gstack commands
6. Select any gstack skill (e.g., gstack:office-hours)
7. Optionally provide input when prompted
8. Watch the skill execute and return results to your OpenCode session
9. Continue working with the results in your OpenCode chat
   This integration maintains full compatibility with existing Claude Code usage while extending gstack's reach to OpenCode users.
   Related to: gstack's mission to provide AI engineering workflows across different agent runtimes.