Add OpenCode support for WinUI skills

[Zhanyuanium (Zhanyuanium)]

Description

OpenCode loads Agent Skills natively from <name>/SKILL.md folders, but there was no guidance for pointing it at the shared plugins/winui/skills/ directory. This PR documents a one-time junction-based setup so OpenCode users consume the same skills (no fork or copy), and makes winui-setup's post-setup guidance harness-neutral while keeping the GitHub Copilot CLI example. No skills are forked or copied, and no existing harness manifests, agents, or parsers change.

Related Issue

Part of #149

Type of Change

• ✨ New skill or feature
• 📝 Documentation

Affected area

• Agent (plugins/winui/agents/)
• Skill: winui-setup
• Tool:
• Plugin metadata (plugin.json, plugins/winui/)
• Repo-level docs / governance

Checklist

• Tested locally on Windows (build + agent invocation if applicable)
• If a skill changed: SKILL.md frontmatter still valid; cross-references to other skills still resolve
• If user-facing: added a bullet to ## [Unreleased] in CHANGELOG.md
• Did not edit any version field in plugins/winui/plugin.json, .github/plugin/marketplace.json, or .claude-plugin/marketplace.json (versions bump only on the staging → main promotion PR — see RELEASING.md)

Screenshots / Demo

Additional Notes

Base branch is staging.

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[Zhanyuanium (Zhanyuanium)]

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