SideChat: temporary snapshot tabs alongside a running thread #12793

While an agent is working, I often want to clarify a term, review a decision with another model, or explore an alternative without interrupting the task or adding the whole detour to its conversation. A new empty thread loses context; asking in the main thread mixes the two tasks.

SideChat would let me:

• Open a temporary tab beside the main thread, with a fixed snapshot of its visible conversation—including output already streamed.
• Explore independently in multiple tabs, each with its own model, reasoning/access controls, draft, and attachments.
• Send a useful answer back explicitly; otherwise keep the main conversation unchanged.
• Keep a side chat as a normal thread or close it. Temporary tabs stay out of navigation.

V2 status: #2829 already includes forks and merge-back. At the checked v2 commit, I found no equivalent temporary, tabbed SideChat panel that snapshots an in-progress conversation. This is a current UI/workflow gap, not a claim that v2 will never add it. Earlier proposals #9436/#9437 were deferred during the rewrite; a draft implementation can illustrate the workflow and later use v2’s backend.

Before: no SideChat surface	After: independent question beside the task

Synthetic demo content in an isolated app. Separate conversation context does not mean a separate filesystem sandbox.