feat: add OpenCode as a supported agent

[Orchardxyz]

Summary

This PR adds OpenCode as a first-class agent in HarnessKit.

What Changed

• registered OpenCode in the canonical agent list and UI metadata
• added OpenCode detection from the config directory, config file, or PATH
• added global OpenCode config discovery
• added OpenCode skill scanning
• added support for local MCP entries defined in opencode.json
• added support for local file-based plugins in the OpenCode plugins directory
• added OpenCode rules and workflow discovery
• updated branding and agent presentation for OpenCode

Scope

This PR intentionally includes:

• agent detection
• global config discovery
• skills
• local MCP servers
• local file-based plugins
• rules
• workflows

This PR intentionally does not include:

• remote MCP servers
• npm-based plugins declared in opencode.json

Screenshots