feat: continue an active agent task in a new branch workspace #148

Summary

Please let users move an ongoing agent-assisted task into an isolated branch workspace while preserving the conversation and the task's progress.

Problem or workflow

A user may begin investigating a problem and then decide the resulting changes belong in a separate workspace. Reopening the task there can require repeating instructions and rebuilding context, and it can be unclear which workspace the agent is using.

Affected area

Branch workspaces and agent session continuation.

Proposed behavior

Connect the existing branch and session capabilities into a guided continuation experience. Preserve the current objective, relevant instructions, progress, and next steps, and clearly show where the resumed agent is working. Keep the original work safe. If the selected agent cannot support continuation, explain that limitation before attempting the move.

For example, a self-hoster investigating a configuration issue should be able to continue the repair in its own branch workspace without restarting the conversation.

Related: #55 and #56. This request focuses on the user's move into a new working workspace, beyond preserving session information or transferring a handoff.