#32825 gallery: a chat and the subagent it opened in the right split, side by side

Leo, 2026-11-02: "WHEN I OPEN A SUBAGENT IT SHOULD GO TO THE RIGHT SPLIT AREA"
Leo, 2026-11-03: "like if they want it open they can just do it on the right split one but otherwise it really doesn't have that much space pretty often"

This adds gallery scenes for #32677 (a subagent opens in the right split) and #32763 (a chat opened in a split starts with Session Info closed).

• agent-pane.subagent-split: two variants of one chat.
  • parent: the chat on the left, with its subagents' line showing each one's state (one running, one done) and Session Info open.
  • child: the pane the app opens on the right on the running subagent (a subagent: reveal).
  • The child play asserts the pane is titled by its task, with no raw id or UUID, shows its own transcript, and keeps Session Info closed.
• agent-pane.subagent-split-pair: the two panes side by side, each a live stage in its own frame. Its play checks that the left pane shows the subagent states with Session Info open, and the right pane shows the title with Session Info closed.

References (cmux-design-references): codex/web/sidebar-parallel-agents, codex/web/multitask, codex-mobile/live/agents-sheet-active-completed, codex-mobile/live/grouped-activity-rows, and Leo's ChatGPT task-thread target (parent on the left with the task's chip, the task's own thread on the right). No ChatGPT task-thread reference is in the index yet.

Not in this PR: a composer in the subagent's pane. acpmux cannot send a subagent a message today (#32809). Sub-sessions already have their own composer.

Observed / Not run

• Observed: vp fmt and vp lint on both files, gallery coverage names these covers, prepush gate. The gallery matrix plays for both entries run through cmux-ci.

Agent session: agent session

🤖 Generated with Claude Code

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Summary by cubic

Adds gallery scenes showing a subagent opening in the right split: the parent chat on the left keeps its subagents' states and Session Info open, while the subagent pane opens on the right titled by its task, with Session Info closed. Covers #32677 and #32763.

• New agent-pane.subagent-split entry with two live variants, parent (the chat) and child (the split-opened subagent pane).
• The child play asserts the pane is titled by its task with no raw id or UUID, shows its own transcript, and does not inherit Session Info open.
• New agent-pane.subagent-split-pair entry renders both panes side by side in separate frames and checks the two behaviors together.
• Not included: a composer in the subagent pane, since acpmux cannot message a subagent yet (#32809).