#22024 fix(cmux-next): restore the agent pane wiring the #21331 merge dropped

Since #21331 merged (10-17 21:49 PDT), four agent pane features that had already landed have been missing on feat-cmux-next:

• The Chief Dock (#21194) never shows. App.tsx stopped passing chiefDock to QuickSurface. The component and its experiment (on by default) still exist, so only the gallery drew it.
• Debug Settings > Turn footer actions (#21344) does nothing. Swift still pushes turnActionsPlacement with the theme, but nothing called configureTurnActionsPlacement, and the [data-turn-actions-placement="left"] rules were gone.
• Tool and edited-file disclosures lost their chat (#21344). ToolRows and EditedFilesCard stopped getting sessionId, so they fall back to one shared bucket, and an expand in one chat can show up in another when ids collide.
• Sent images lost their 72px slot and reply images their fade (#21383). The placeholder span still renders, at zero size, so the layout shift it was added to stop came back.

The merge itself was faithful: a three-way merge of its parents gives the same tree. The cause is the branch. e124e93b3e17 ("rebase focused streaming motion after Chief Dock") removed these hunks relative to its base 50a0c607aee, apparently from a tree that predated them. This PR re-applies only those hunks. #21331's own work stays as it is: the follow-bottom deadband, THREAD_TOP and overflow-anchor. The branch also dropped the effort track motion, but #21512 already put that back.

webviews/test/agent-pane-merge-restore.test.ts checks the two CSS rules through JSDOM's parsed stylesheet. It checks the App.tsx wiring as source text: the placement call, sessionId on both rows, and the dock prop with its experiment gate. That's a guard against another stale rebase, not a render test.

• Red on the first commit: 5 of 5 fail.
• Green on the second.

vp check passes, and so do sent-image-lifecycle, turn-actions-placement and transcript-disclosure. The type-scale and layers guards pass. I didn't take screenshots or test in the app.

Changelog

Fixed: The Chief Dock, the left turn-footer option and the sent-image placeholder work again in agent chats.

🤖 Generated with Claude Code

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Summary by cubic

Restores four agent pane features that the #21331 merge dropped on feat-cmux-next, because the branch was rebased onto a tree without them. A new test guards against another stale rebase.

Bug Fixes

• The Chief Dock mounts again behind its experiment (#21194).
• configureTurnActionsPlacement runs on theme push and the left-placement CSS rules are back (#21344).
• Tool and edited-file rows pass the chat's sessionId to their disclosures, so expands no longer leak across chats (#21344).
• The 72px sent-image placeholder and the reply-image fade are restored (#21383).