Feature Request: Add `cwd` field to `AgentDefinition` to support per-agent working directories

# Feature Request: Add `cwd` field to `AgentDefinition` to support per-agent working directories

## Summary

When building multi-agent systems where different agents are specialized for different project workspaces, there's currently no way to set a per-agent working directory (`cwd`) in `AgentDefinition`. All subagents inherit the parent `query()` call's `cwd`.

## Use Case

I'm building a multi-agent orchestration layer where each agent is a domain expert locked to its own project directory (e.g., a vehicle diagnosis agent → `/home/code/vehicle-diagnosis`, a frontend agent → `/home/code/frontend`). Each workspace has its own `CLAUDE.md`, memory files, and project context.

When the main orchestrator spawns these agents as teammates, they need to operate in their respective workspaces — not the orchestrator's working directory.

## Current Limitation

`AgentDefinition` has no `cwd` field:

```ts
type AgentDefinition = {
description: string
prompt: string
tools?: string[]
model?: string
mcpServers?: AgentMcpServerSpec[]
// no cwd ❌
}
```

So calling:

```ts
query({
agents: {
'diagnosis-agent': {
description: 'Vehicle diagnosis expert',
prompt: '...',
// no way to set cwd: '/home/code/vehicle-diagnosis'
}
}
})
```

means the subagent runs in the parent's `cwd`, reads the wrong `CLAUDE.md`, and loses its specialized context.

## Proposed Solution

Add an optional `cwd` field to `AgentDefinition`:

```ts
type AgentDefinition = {
description: string
prompt: string
tools?: string[]
model?: string
mcpServers?: AgentMcpServerSpec[]
cwd?: string // ← working directory for this subagent ✅
}
```

## Notes

I've reviewed the Claude Code source code and can see `runWithCwdOverride()` is used internally when spawning pane-based teammates — this suggests the infrastructure already exists. The gap is that the SDK's public `AgentDefinition` API doesn't expose it.