feat: support unix domain socket

[prashanthpai]

Unix domain socket support is particularly useful in sandboxed environments where we may not want to expose ports.

• adds --unix option to server
• adds support to pass socket file path to opencode attach on the client

Example:

Server:

$ ./packages/opencode/dist/opencode-darwin-arm64/bin/opencode serve --unix /tmp/opencode.sock

Client:

$ ./packages/opencode/dist/opencode-darwin-arm64/bin/opencode attach unix:///tmp/opencode.sock

Monitor usage:

$ watch lsof /tmp/opencode.sock

Closes: #2119
Similar PR: #2120

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

• PR feat(server): add unix domain socket support #2120: feat(server): add unix domain socket support
  feat(server): add unix domain socket support #2120

This PR appears to be directly related to the current PR #9668 as it also addresses unix domain socket support for the server. Both PRs are implementing similar functionality for handling socket-based connections rather than port-based ones.