docs: document non-interactive MCP add

[dannyward630]

Summary

• document non-interactive opencode mcp add usage
• add a remote MCP server example using --url and --header
• list the related --url, --header, and --env flags

Closes #32259

Verification

• bun run build from packages/web
• full pre-push bun turbo typecheck

[github-actions]

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[github-actions]

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

Potential Duplicate Found:

• PR docs(cli): document non-interactive mcp add flags #32283: docs(cli): document non-interactive mcp add flags
  • docs(cli): document non-interactive mcp add flags #32283

This PR appears to be addressing the same issue. Both PRs are documenting non-interactive MCP add functionality with the same flags (--url, --header, etc.). PR #32283 was likely created for the same issue (#32259) and covers the same scope. You may want to check if one of these PRs can be closed in favor of the other to avoid duplicate efforts.

[github-actions]

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