Support interactive input variables (${input:...}) for plugins in GitHub Copilot CLI

### Describe the feature or problem you'd like to solve

GitHub Copilot CLI currently has no support for interactive input variables (for example, ${input:...} in .mcp.json). This makes it difficult for plugins to securely prompt users for values such as API keys, URLs, or other configuration at runtime, reducing compatibility with existing MCP configurations.

### Proposed solution

Introduce an interactive input mechanism for Copilot CLI similar to VS Code's ${input:...} support.

- Support resolving ${input:...} variables when a Copilot CLI session starts, prompting the user for required values.

- Provide a dedicated CLI configuration section (similar to Claude Code's userConfig https://code.claude.com/docs/en/plugins-reference#user-configuration) where plugins can declare user-configurable parameters that are prompted interactively and made available during execution.

For example:

```json
{
"inputs": [
{
"id": "github_token",
"type": "password",
"description": "GitHub Personal Access Token"
}
]
}

```

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start with the .mcp.json example and the session-start flow described in the issue, then trace how plugin configuration is currently loaded. Done means plugins can declare inputs, the CLI prompts for required values at session start, and resolved values are available during execution.

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