Add opencode-mdplugin-engineer agent for asset bundle packages

[diegohb]

Adds specialist agent for creating npm packages that bundle OpenCode skills and commands.

What this adds:

• Agent that guides developers through creating distributable npm packages
• Supports two distribution approaches:
  • Copying (consumer-editable): Assets copied to .opencode/ directory
  • Path registration (black-box): Assets stay inside npm package
• Comprehensive templates for SKILL.md and command.md assets
• Detailed code examples for both approaches

Why this matters:

• Distinct from opencode-plugin-engineer (which builds plugins with hooks/tools)
• Enables plugin-less OpenCode extensions via asset packages
• Addresses need for distributable skill/command bundles
• Provides clear guidance on distribution trade-offs

[diegohb]

@michalwarda After a day or two of wrestling with how to package these up I landed on the realization that the plugin mechanism is the only way to do it "natively". I fought with the idea of adding code to what otherwise is just simple MD files but here it is. It includes the optionality to choose whether the skills will be copied over or not to the end-consumer (instead of hiding in node_modules).

To be honest, I haven't tested this at all and it was entirely vibe-coded but the approach and template for doing the skills/commands is solid - that's what I spent time on.. at end I just didn't wanna horde that in my random session transcript that will dissappear.

I can test it thoroughly if you are interested in the PR but I wanted to make sure before I spend more time on it.