feat(github-action): Support remote server connection via --attach

[dl-alexandre]

Issue for this PR

Closes#15801

Type of change

• New feature

What does this PR do?

Adds support for connecting the GitHub Action to a remote OpenCode server via the --attach functionality. This enables the Action to serve as a "remote trigger" that executes prompts on persistent servers holding deep architectural context, warm caches, or specialized MCP tool configurations.

Changes made:

1. action.yml: Added two new optional inputs:

   • server_url: URL of the remote OpenCode server (e.g., https://agent.acme.com:4096)
   • server_password: Password for authentication (should be passed via GitHub secrets)

2. index.ts: Modified createOpencode() function to:

   • Check if server_url is provided and connect to remote server via SDK
   • Add Authorization: Bearer <password> header when server_password is provided
   • Validate that server_password cannot be used without server_url
   • Return noop close() function for remote servers (skip process kill)
   • Maintain existing local server behavior as default when no server_url provided

Why these changes work:
The OpenCode CLI already supports --attach flag for remote execution. This PR exposes that functionality in the GitHub Action by conditionally creating an SDK client connected to the remote URL instead of spawning a local opencode serve process. The implementation uses the existing SDK's createOpencodeClient() with custom headers for authentication.

Use cases enabled:

• Persistent context: Run PR reviews on servers that "know" the codebase long-term
• Hardware offloading: Move heavy computation from GitHub runners to high-performance private servers
• Unified agent state: Share session history between automated tasks and human TUI sessions

How did you verify your code works?

• Verified TypeScript compiles without errors (typecheck passes)
• Confirmed logic handles both remote and local modes correctly:
  • When server_url provided: connects to remote with optional auth
  • When not provided: maintains existing local behavior
• Tested validation logic: password without URL throws descriptive error
• Reviewed cleanup behavior: remote servers don't attempt process kill

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[rekram1-node]

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