[Feature]: First-class OpenCode support for WinUI skills

What problem are you trying to solve?

The winui plugin exposes its capabilities primarily to GitHub Copilot CLI (agents via .agent.md, skills via the plugin manifest), with separate adapters for Claude Code and Codex. OpenCode users currently have no first-class path: they can't link the shared skills, parse OpenCode sessions in winui-session-report, or run the winui-dev orchestrator agent. A developer who has standardized on OpenCode therefore can't get the same end-to-end WinUI 3 build flow that Copilot CLI provides.

Proposed solution

Add first-class OpenCode support to the winui plugin, delivered as three independent, independently mergeable changes (each targeting staging):

• Skill discovery — link the shared plugins/winui/skills/ into OpenCode's skills directory (no fork/copy). → #146
• Session report — teach Analyze-Session.ps1 to parse OpenCode sessions via opencode export. → #147
• Orchestrator agent — expose winui-dev to OpenCode (opencode run --agent winui-dev), loading winui-dev-workflow and winui-design on demand. → #148

Each PR is self-contained and can be reviewed/merged on its own; together they give OpenCode the same discover → analyze → orchestrate loop the other harnesses already have.

Alternatives considered

• Single mega-PR — rejected: too large to review in a slow-review repo; a defect in one area would block the whole feature.
• Sequential stacked PRs — rejected: each layer waits on the previous review, multiplying the already-slow latency.
• Skill-only, no orchestrator agent — rejected: OpenCode can map skills, but without an agent the "do the build yourself" flow the winui-dev agent encodes is lost.

Additional context

• PRs #146 (skill discovery), #147 (session report), #148 (orchestrator agent) all target staging.
• Cross-harness precedent: the plugin already ships .claude-plugin/ and .codex-plugin/ adapters; this extends the same pattern to OpenCode.

This umbrella issue is split across #146, #147, and #148, covering plugins/winui/skills/, Analyze-Session.ps1, and the winui-dev agent. Review those linked changes first, then verify that each independently adds its stated OpenCode integration and that the complete discover, analyze, and orchestrate flow works.