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

Start at the TypeScript ResumeSessionConfig and the resumeSession(sessionId, ...) entry point, then compare its customAgents handling with createSession. Trace the session.resume path and timeout error formatting. Done means customAgents behavior on resume is supported or explicitly rejected, and timeout messages show a numeric value instead of [object Object].

Written by the indexing model from the issue text.