Feature Request: Session Export and Replay for Agent Debugging

**Title**

Feature Request: Session Export and Replay for Agent Debugging

## Description

### Problem

When building agents with the Copilot SDK, debugging complex multi-turn conversations can be difficult.

Developers often encounter situations where:

* An agent behaves unexpectedly after several turns.
* A tool invocation produces an incorrect result.
* Context accumulation affects later responses.
* Bugs only appear after a specific sequence of interactions.

Currently, reproducing these issues typically requires manually recreating the entire conversation flow.

### Proposed Solution

Add session export and replay capabilities to the Copilot SDK.

#### Session Export

Allow developers to export a session including:

* User messages
* Assistant messages
* Tool calls
* Tool results
* MCP interactions
* Session metadata
* Model configuration

Example:

```json
{
"sessionId": "...",
"messages": [...],
"toolCalls": [...],
"metadata": {...}
}
```

#### Session Replay

Provide APIs to replay exported sessions for debugging and testing.

Example:

```typescript
const session = await sdk.loadSession("./session.json");

await session.replay();
```

#### Optional Features

* Replay until a specific turn
* Step-by-step execution
* Compare outputs across SDK versions
* Compare outputs across models
* Deterministic replay mode

### Use Cases

#### Agent Development

Reproduce user-reported failures without requiring users to recreate conversations.

#### Regression Testing

Verify that SDK upgrades do not change agent behavior unexpectedly.

#### Tool Debugging

Inspect exactly how tool calls and results affected agent decisions.

#### Benchmarking

Compare responses across different models using the same session history.

### Benefits

* Easier debugging
* Better reproducibility
* Improved regression testing
* Faster issue triage
* Better developer experience for agent builders

### Alternatives Considered

* Manual logging of messages
* Custom session serialization
* Third-party observability platforms

These approaches require significant additional implementation and are not standardized across SDK users.