Syntax Parity: Support VS Code Custom Agent definitions (e.g., handoffs) in Copilot CLI

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

The Copilot CLI currently fails to parse valid VS Code custom agent definitions (e.g., the handoffs field), resulting in "malformed frontmatter" errors.

### Proposed solution

Currently, there is a syntax discrepancy between the VS Code extension and the Copilot CLI regarding Custom Agent definitions.

When I load a workspace containing Custom Agents created for VS Code (specifically those using the handoffs field in the frontmatter), the Copilot CLI fails to parse them, returning "custom agent markdown frontmatter is malformed" and "unknown field ignored: handoffs" warnings.

I would like to solve this fragmentation by having the CLI support the same frontmatter schema as VS Code. This would allow developers to maintain a single, unified set of agent definitions ("single source of truth") that works seamlessly across both environments.

### Example prompts or workflows

_No response_

### Additional context