Session forking — branch a conversation into parallel sessions with shared context

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

When working on a multi-step task, I often hit a natural junction where two independent problems emerge. Today I have to choose: finish one before starting the other, or lose my current context by switching sessions.

### Proposed solution

A `/fork` command (or similar) that creates a new parallel session initialized with the current session's conversation history and context. Both the original and forked session remain independently interactive — I can switch between them with `/resume` or work in
separate terminals.

### Example prompts or workflows

Mid‑refactor in an app, I discover a bug in a shared library.
/fork → fix the library in the new session, continue the refactor in the original.

While investigating a test failure, I realize the fix requires both a code change and an infra change.
/fork → one session handles code, the other handles infra, both with full context of the investigation.

Planning a feature, I want to prototype two different approaches simultaneously.
/fork twice → three sessions exploring different designs, all starting from the same analysis.

Working in a tenant repo, I identify a needed change to an upstream template repo.
/fork → plan/implement the upstream PR in the new session while continuing tenant work in the original.

Debugging a frontend issue, I realize the backend API also needs changes.
/fork → one session for frontend, one for backend, both understanding the full problem.

### Additional context

_No response_

Start by tracing the existing session and /resume entry points, then compare them with the proposed /fork workflow in this issue. Define how a fork copies conversation context while remaining independently interactive, and verify that the original, forked, and multiply forked sessions can be resumed or used from separate terminals.

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