[FEATURE]: Tab groups with "send to group" for comparing sessions side by side #51523

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When I want to compare how different models handle the same question, I open one session tab per model and paste the same prompt into each tab. With three or four tabs this is slow and error prone (edits to the prompt have to be repeated in every tab).

Proposal: let session tabs be gathered into a named group, and add a "Send to tab group" action to the composer.

• Grouping is done after the sessions exist: a "Tab group..." command on the current tab offers existing groups, creating a new one by typing a name, or removing the tab from its group. Grouped tabs get a small colour bar in the tab strip.
• "Send to tab group" (mod+shift+enter, also in the command palette) submits the prompt to the current session exactly as Enter does, and sends the same text and attachments to every other session in the group. Each target keeps its own agent and model; busy sessions receive it as a queued follow-up after their current turn ends; a toast lists what was sent, queued or failed.
• Groups are UI state only (persisted per window next to the tab list). No server or API changes.

This is related to #44589 (grouping tabs by project for organisation); the ask here is user-defined groups that act as a send target.

I have a working implementation on v2 for review: https://github.com/iyernaveenr/opencode/tree/naveen_r_iyer/v2-tab-groups (tab store + dialog + composer fan-out, with tests). Not opening a PR until the direction is confirmed.