Allow users to "alt+tab" to agent-spawned shell sessions for interactive tool use

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

When the Copilot CLI spawns a shell process, I have no way to see or interact with that shell directly, in the event it decides to use one of my tools that functions as a terminal ui. Just today, copilot instructed me to use arrow keys, this is guidance that I cannot follow.

### Proposed solution

Add a mechanism for the user to switch to or "alt+tab" to a running agent spawned shell session.

Perhaps, a `/shell ` command that swaps focus to the live session, or a tab or split-pane view showing active shell sessions the agent has started.

### Example prompts or workflows

The agent is great at knowing what to run but sometimes the best tool for the job is interactive. Right now, the agent must either avoid interactive tools entirely or attempt to puppet them through a text proxy both are worse outcomes than just letting me take the wheel for 5 seconds.

Example workflows this enables:
1. "Find me the config file for X"
2. "Debug this test failure"
3. "Resolve these merge conflicts"
4. "Help me set up this project"
5. "Show me what's running on this port"

### Additional context

_No response_

Start by examining how the Copilot CLI spawns and tracks shell processes, then trace how interactive tool use is currently mediated. Compare the proposed /shell command with tab or split-pane session switching; done means a user can take control of a live agent-spawned shell and return to the agent safely.

Written by the indexing model from the issue text.