FLEET: resumeSession() with customAgents errors with malformed timeout message "[object Object]ms"

## Summary

Calling `client.resumeSession(sessionId, { customAgents: [...] })` on a live session produces a timeout error with a malformed message: `"Timeout after [object Object]ms"`. This reveals two bugs: (1) `resumeSession` with `customAgents` fails, and (2) the timeout value is being stringified via implicit `.toString()` instead of extracting the numeric value.

## Steps to Reproduce

```javascript
import { CopilotClient, approveAll } from '@github/copilot-sdk';

const client = new CopilotClient();
await client.start();

const session = await client.createSession({ onPermissionRequest: approveAll });
const sessionId = session.id;
await session.destroy();

try {
const resumed = await client.resumeSession(sessionId, {
onPermissionRequest: approveAll,
customAgents: [{
name: 'post-creation-agent',
displayName: 'Post Creation Agent',
description: 'Added during resume',
prompt: 'You are a post-creation agent.',
tools: ['view'],
infer: false,
}],
});
} catch (err) {
console.error(err.message);
// → "Timeout after [object Object]ms waiting for session.idle"
}
```

## Observed Behaviour

Spike data (2026-03-17):

```json
{
"resumeSucceeded": false,
"resumeError": "Timeout after [object Object]ms waiting for session.idle",
"answer": "NO/ERROR"
}
```

## Bug 1: resumeSession with customAgents fails

The TypeScript type `ResumeSessionConfig` accepts `customAgents`, but the CLI backend appears to reject or ignore it and the session never reaches `idle`. Either:
- `customAgents` is not supported on resume (the type should exclude it), or
- There is a backend defect handling `customAgents` in `session.resume`

## Bug 2: Malformed timeout error message

`"Timeout after [object Object]ms"` is a string formatting bug. The timeout config value (likely `{ ms: number }` or similar) is being coerced to string via `.toString()` instead of extracting the numeric value.

Expected: `"Timeout after 30000ms waiting for session.idle"`

## Workaround

Only set `customAgents` at initial `createSession` time. Do not attempt to add agents via `resumeSession`:

```javascript
// ✅ Works
const session = await client.createSession({
onPermissionRequest: approveAll,
customAgents: [{ name: 'my-agent', ... }],
});

// ❌ Errors with malformed timeout
const session = await client.resumeSession(id, {
customAgents: [{ name: 'my-agent', ... }],
});
```

## Environment

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