FLEET: fleet.start() ignores customAgents — always dispatches built-in agent types

## Summary

When a session is created with `customAgents` defined, calling `rpc.fleet.start()` **ignores the registered custom agents entirely**. Sub-agents are dispatched as built-in types (`explore`, `general-purpose`, etc.), never as the custom agents registered in the session config.

## Steps to Reproduce

```javascript
const session = await client.createSession({
onPermissionRequest: async () => ({ approved: true }),
customAgents: [{
name: 'my-researcher',
displayName: 'My Researcher',
description: 'A custom agent that only reads files.',
prompt: 'You are a focused research agent. Only read files.',
tools: ['view', 'grep'],
infer: false,
}],
});

// Confirm registered:
const agentList = await session.rpc.agent.list();
// → { agents: [{ name: 'my-researcher', ... }] } ✓ registered

await session.rpc.fleet.start({ prompt: 'Research X in parallel' });

session.on('subagent.started', (data) => {
console.log(data.agentName);
// → 'explore' ← always built-in, never 'my-researcher'
});
```

## Observed Behaviour

Fleet dispatches `explore` built-ins regardless of `customAgents` config. The custom agent is visible in `rpc.agent.list()` immediately before the fleet call, confirming registration is not the issue.

Spike data (2026-03-17):
```json
{
"agentNamesObserved": ["explore", "explore"],
"subagentCount": 2,
"answer": "PARTIAL — dispatched built-in names, not custom names"
}
```

## Expected Behaviour

One of:
1. `fleet.start()` uses custom agents when registered in the session
2. `fleet.start()` accepts `agentNames: string[]` to specify which agents to dispatch
3. Documentation clarifies fleet only uses built-in types (so callers know not to expect otherwise)

## Impact

- Custom agent tool restrictions (`customAgents[n].tools`) cannot be applied to fleet workers
- Custom system prompts are ignored during fleet
- Fleet workers always get unrestricted built-in agent capabilities regardless of session config

## Environment

- SDK: `@github/copilot-sdk` v0.1.32
- CLI: latest
- Node: v24.13.1