[FEATURE]: add OpenCode skills configuration and generation pipeline

[Piping]

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Add first-class OpenCode integration for this repository so OpenCode can discover and load all skills from skills/ consistently with other agents.

Proposed scope:

• Add generated root config file: opencode.json
• Include all discovered skills from skills/*/SKILL.md
• Include AGENTS context entry (agents/AGENTS.md)
• Include MCP server settings aligned with .mcp.json / gemini-extension.json
• Add script-level generation/check flow and wire into scripts/publish.sh
• Document OpenCode usage in README and dedicated docs

Benefits:

• Keeps OpenCode setup reproducible and consistent with Claude/Cursor/Gemini integrations
• Avoids manual drift via generated artifacts and --check validation
• Reduces onboarding friction for contributors and users adopting OpenCode