feat(marketplace): Add GitHub-based agent marketplace for OpenCode

[mporracindie]

Summary

• Introduces a decentralized agent marketplace using GitHub repositories as registries, similar to Claude Code's plugin marketplace
• Supports both public and private repositories with a flexible authentication chain
• Provides auto-discovery fallback for Claude Code compatibility (repos without registry.json)
• Integrates seamlessly into the TUI with search, browse, and installation capabilities

Test plan

• Add a public GitHub repo as marketplace source in opencode.jsonc
• Open TUI and access "Browse marketplace" from command palette (ctrl+p)
• Verify agents are listed from the configured source
• Test search functionality across agent names, descriptions, and tags
• Install an agent to project scope (.opencode/agent/) and verify it appears in agent list
• Install an agent to global scope (~/.config/opencode/agent/) and verify it works
• Test with a Claude Code marketplace repo (no registry.json) to verify auto-discovery works
• Test private repo access with GITHUB_TOKEN environment variable
• Verify cache invalidation with refresh (ctrl+r in marketplace dialog)