Agent-created child sessions ignore a preset plugin-provided custom agent and fall back to Default agent

### Summary

When an **agent** creates a child session and presets its agent (via the session-creation tool that accepts an `agent` parameter), a **plugin-provided custom agent** is not applied. The new session silently starts on **Default agent** instead. Manually selecting the same agent from the session's agent dropdown works fine, so the agent itself is valid and loadable — only the agent-created "start this session in agent X" path fails for plugin-contributed agents.

### Environment

- GitHub Copilot app (agent-native desktop), Windows
- App version: `1.0.21`
- Custom agent provided by an **installed plugin** (appears as a selectable entry in the agent picker)

### Steps to reproduce

1. Install a plugin that contributes a user-invocable custom agent (the agent shows up in the agent dropdown).
2. Have an agent create a child session and set the preset agent to that plugin agent (the `agent` field on the session-creation / kickoff call).
3. Let the session start (with or without a kickoff prompt).
4. Open the created session and check its agent dropdown.

### Expected

The created session starts bound to the specified plugin-provided agent (dropdown shows that agent; the session runs under that agent's persona/workflow).

### Actual

The session starts on **Default agent**. The preset agent is silently ignored — no error or warning is surfaced. The kickoff prompt (if any) runs under the Default agent instead of the intended one.

### What I tried (identifier is not the problem)

I attempted the preset with three identifier forms; all three resulted in Default agent:

1. Bare agent name — ``
2. Plugin-qualified — `:`
3. File-basename-qualified — `:.agent`

I verified the identifier is correct:
- The plugin's `plugin.json` `name` matches the namespace used.
- The agent's frontmatter `name` matches the label shown in the agent picker.
- The agent is present and selectable in the UI dropdown, and **manually selecting it works**.

This points to the agent-created preset path not resolving/applying **plugin-contributed** agents (built-in/workspace custom agents may behave differently), rather than a wrong identifier.

### Impact

A workflow that has an agent spin up child sessions preset to a specific plugin agent can't work — every session must have its agent selected by hand in the UI, defeating unattended/parallel session orchestration. Silent fallback to Default also means automated runs can appear to "work" while actually executing under the wrong agent, producing misleading results.

Start by tracing the session-creation or kickoff call's agent parameter and compare its resolution path with the agent dropdown, using the reproduction steps with a plugin-provided agent. Done means a child session created with the plugin agent opens with that agent selected and runs its kickoff prompt under it instead of silently falling back to Default.

Written by the indexing model from the issue text.