Add ability to fork/clone a session to enable parallel task branches with shared context

### Describe the feature or problem you'd like to solve

During long-running Copilot CLI sessions, it's common to discover a new significant task while deep in an existing one. Today, the options for handling this are unsatisfying: - **Stay in the same session**: Going deep on the new task pollutes the context window and makes it harder to resume the original task. The conversation history becomes a tangled mix of two unrelated efforts. - **Start a new session (`/new`)**: The new task loses all relevant context from the current session — file understanding, architectural decisions, prior investigation — even though that context is directly useful for the new work. - **Compact and switch (`/compact`)**: Compaction is lossy. Important details about the in-progress task (partial plans, nuanced decisions, tool output) are often summarized away, making it difficult to resume with full fidelity.

### Proposed solution

Add a `/fork` slash command (and corresponding `--fork` CLI option) that duplicates the current session into two independent sessions sharing the same conversation history up to the fork point.

```
/fork [NAME]
```

- Creates a new session that is an exact copy of the current session's context (conversation history, checkpoints, working directory, permissions, etc.) up to the point of the fork.
- The current session remains active and unchanged.
- The new (forked) session is given an optional name, or auto-named (e.g., `-fork-1`).
- After forking, the user can switch to the new session with `/resume` to work on the new task, and later `/resume` back to the original session to continue the earlier work.
- Both sessions evolve independently from the fork point onward.

### Example prompts or workflows

_No response_

### Additional context

_No response_