feat(web): add temporary side chats with context snapshots

[rohan-patnaik]

A side question should not interrupt a running task or add an entire exploratory exchange to its context. This adds temporary SideChat tabs beside the main thread, opened from the panel launcher, /side, or Ctrl+Alt+S.

Each tab captures a fixed conversation snapshot, including partial streamed output, and uses an independent provider session. Tabs have separate models, reasoning/access controls, drafts, and attachments. Send to main chat explicitly dispatches a completed answer using the parent’s settings; Keep promotes the tab to a normal thread and Close discards it. The pane resizes and preserves inactive tabs.

Refs #12793.

Draft for workflow review: #2829 has fork/merge-back support, but the checked v2 branch has no equivalent temporary tabbed panel for an in-progress snapshot. #9436/#9437 were deferred because the orchestration/provider layers are being rewritten. This implementation targets current main to make the interaction runnable; its snapshot/session plumbing should be reconciled with v2 before merging. It does not imply v2 lacks forks or merge-back.

Before	After

Second independent tab and explicit send-back

Validation: 276 focused tests passed across server, web, and client-runtime. Web, server, and client-runtime typechecks passed; scoped lint completed with warnings. Tests cover streaming snapshot isolation and restart, attachment ownership, all six provider adapters through test doubles, oversized context, temporary navigation, tab/draft independence, Keep, and send-back success/failure with the parent’s model. Cua Driver exercised tab creation/closing, independent model display, draft retention, and the isolated screenshots. Screenshots use synthetic fixtures; no live provider calls were used as proof.

The web UI is shared by desktop; shared navigation hides temporary threads on all clients. Native mobile has no SideChat panel in this PR and was not exercised. Requests stay environment-scoped for remote connections. Snapshots are persisted for reload recovery and contain visible history, not hidden reasoning or live tool state. Side sessions share the workspace; they are not filesystem sandboxes. Hidden side chats retain worktree ownership and become visible if their parent disappears.

Implemented with GPT-6 Astra (medium), Codex harness in T3 Code.

[juliusmarminge]

Closing as superseded: the related feature request #12793 was closed as a duplicate of discussion #7311. Please continue on that discussion (and related #8343 / #11818); earlier SideChat drafts were deferred during the rewrite (#9436/#9437).