Custom agent frontmatter `model` field rejects array syntax — incompatibility between Copilot CLI and VS Code Copilot Chat

### Describe the bug

When a custom agent `.agent.md` file uses an **array** for the `model` frontmatter field (as supported by VS Code Copilot Chat), the GitHub Copilot CLI refuses to load the agent and throws a parse error.

**Error message from Copilot CLI:**
```
The following agents failed to load:
.github/agents/Coordinator.agent.md: custom agent markdown frontmatter is malformed: model: Expected string, received array
```

**Agent frontmatter that triggers the error:**
```yaml
---
name: Coordinator
description: Pure managerial agent that orchestrates implementation by delegating all technical work to specialized subagents.
argument-hint: Coordinate a plan, phase, feature, or code change
tools: [vscode, execute, read, agent, search, web, todo]
model: [GPT-5.4 (copilot), Claude Sonnet 4.6 (copilot)]
---
```

The `model` field being an array is intentional — it allows the agent to declare multiple model fallbacks or alternatives. VS Code Copilot Chat loads and uses this agent correctly. However, the Copilot CLI strictly expects `model` to be a **string**, causing a schema validation failure and preventing the agent from loading entirely.

### Affected version

_No response_

### Steps to reproduce the behavior

1. Create a custom agent file at `.github/agents/MyAgent.agent.md` with the following frontmatter:
```yaml
---
name: MyAgent
description: An agent with multiple model options
model: [GPT-5.4 (copilot), Claude Sonnet 4.6 (copilot)]
---
```
2. Run any `copilot` CLI command that triggers agent loading (e.g. `gh copilot suggest` or open a session that scans `.github/agents/`)
3. Observe the error output on checking /agent

### Expected behavior

The Copilot CLI should accept the `model` field as either a string **or** an array of strings, consistent with how VS Code Copilot Chat parses the same `.agent.md` frontmatter. When an array is provided, the CLI could use the first entry as the primary model, or support model fallback/selection semantics.

At minimum, the schema validation for `model` should align between Copilot CLI and VS Code Copilot Chat to avoid agents that work in one environment failing to load in the other.

### Additional context

_No response_

Beginne damit, das Problem mit .github/agents/MyAgent.agent.md zu reproduzieren und das Laden von Agents mit /agent oder einem anderen Befehl zu überprüfen, der .github/agents/ scannt. Verfolge die Frontmatter-Validierung für benutzerdefinierte Agents und vergleiche die akzeptierte Modellstruktur mit VS Code Copilot Chat. Die Aufgabe ist erledigt, wenn Frontmatter für das Modell mit einem Array-Wert ohne den gemeldeten Parse-Fehler geladen wird und vorhandene Agents mit einem Modell als String weiterhin funktionieren.

Vom Indexierungsmodell aus dem Issue-Text verfasst.