Add OpenCode Agent SDK dependency to server workspace

[Copilot]

What Changed

To address the request to add OpenCode Agent SDK ASAP with minimal scope, this PR adds the SDK as a direct dependency in the server package only.

• Dependency update
  • Added @opencode-ai/sdk to apps/server/package.json.

{
  "dependencies": {
    "@anthropic-ai/claude-agent-sdk": "^0.2.77",
    "@opencode-ai/sdk": "^1.2.27"
  }
}

Why

The issue asks for OpenCode Agent SDK integration with minimum change.
Adding the package at the server boundary makes the SDK available for follow-up provider wiring while keeping runtime behavior unchanged and PR scope tightly constrained.

UI Changes

No UI changes.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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