Session forking with preserved context

[doyled-it]

Motivation

agent-deck supports forking a session so the new session inherits full conversation history. Useful for branching exploration — try two approaches from the same point.

Scope

• Action on a session: 'Fork' (key binding TBD).
• Implementation depends on runner: for Claude Code, copy the transcript file into a new session dir and launch with --resume or equivalent. For other runners, define per-runner fork strategy or mark as unsupported.
• New session inherits parent's runner, model, working dir; gets a new tmux name and new branch (optional).
• Track parent_session_id on the session row to render a fork tree.

Depends on

• Pluggable runners issue (fork is a per-runner capability).

Open questions

• UI for visualizing fork ancestry — sidebar tree vs. flat list with parent column.
• Whether to fork worktree as well (probably yes, default to a new branch off parent's branch).