disable-model-invocation dosen't work

- Copilot Chat Extension Version: unknown
- VS Code Version: 1.132.0-insider
- OS Version: Windows 11
- Feature (e.g. agent/edit/ask mode): Agent mode
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): Qwen3.6:35b
- Logs: (attached)

## Description

Agents with `disable-model-invocation: true` are being invoked by other agents despite the setting. Specifically, the **Orchestrate** agent (`.github/agents/Orchestrate.agent.md`) has `disable-model-invocation: true` set, which should prevent any model from invoking it directly. However, other agents occasionally invoke Orchestrate instead of routing to the correct subagent.

## Steps to Reproduce

1. Configure an agent file with `disable-model-invocation: true` (e.g., `Orchestrate.agent.md`)
2. Trigger a task that should route to a specific subagent (e.g., Code, Git, Memory)
3. Observe that the model invokes Orchestrate instead of the intended target subagent

## Expected Behavior

Agents with `disable-model-invocation: true` should never be invoked by the model. The model should respect this flag and route to the correct subagent based on the agent-mapping rules.

## Actual Behavior

Orchestrate (and potentially other agents with this flag) is occasionally invoked, bypassing the intended routing logic.

## Agent Configuration

**Orchestrate agent** (`.github/agents/Orchestrate.agent.md`):
```yaml
---
name: Orchestrate
description: Passthrough delegator. Forwards every user request verbatim to a single subAgent tool call. Does not think, plan, ask questions, or perform any work itself — only delegates.
argument-hint: The user's original request (passed as-is to the subagent)
target: vscode
disable-model-invocation: true
tools: [agent]
agents: ['GitAgent', 'CodeAgent','MemoryAgent', 'Planer', 'Explore', 'Ask']
---