feat(opencode): Command Support on Config with {cmd:}

[dbpolito]

Issue for this PR

Closes #12710

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Add support for add {cmd:}, which makes things like this possible:

  "mcp": {
    "github-test": {
      "type": "remote",
      "url": "https://api.githubcopilot.com/mcp/",
      "enabled": false,
      "headers": {
        "Authorization": "Bearer {cmd:gh auth token}",
      },
    },
  },

How did you verify your code works?

Added tests, manual testing

Screenshots / recordings

Connected using config above

Checklist

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