#17504 cmux-next agent pane: one sidebar, drop the pane's own chat list

The Agent pane had a second sidebar inside it: an icon rail (Home, chats, history, more) plus New chat, Search and a session list. #17348 collapsed it by default, but the header toggle and the new tab page's "Show all" still opened it next to the window sidebar. cmux-next has one sidebar, so the pane no longer draws one.

• The pane no longer mounts SessionSidebar, its narrow-pane overlay and scrim, or the header toggle. The compact header (title, status, summary, link, Continue in) stays.
• "Show all" on the new tab page opens Search chats (the pane's existing Cmd-K palette) instead of the removed list.
• SessionSidebar.tsx stays for the prototype shell, which still uses it.

New chat, search and a Recents list move into the window sidebar in a follow-up PR in this lane.

Testing

• singleSidebar.test.tsx replaces the overlay test. It fails on the base commit (the pane still draws .acpmux-sidebar) and passes here; it also checks that picking a chat from Search chats selects it. Red and green were run with bun test src/agent-session/acpmux/singleSidebar.test.tsx.
• singleSidebar, searchChatsCommand, NewTabPage and homeLists tests: 18 pass, 0 fail. These were run locally, one file set at a time under a memory cap, because PR CI into feat-cmux-next doesn't run webviews tests.
• build-agent-pane-web.sh --check reports the committed bundle is current.
• Fleet build and before/after captures are posted in a comment.

Changelog

• Removed: the Agent pane's own chat sidebar; agent chats live in the window sidebar.

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Summary by cubic

Removes the agent pane's own sidebar (icon rail, session list, header toggle) since cmux-next has a single window sidebar that already owns agent chats. The compact header stays, and "Show all" on the new tab page opens Search chats instead of the removed list.

• Replaces the sidebar overlay test (SessionSidebarOverlay.test.tsx) with singleSidebar.test.tsx, which asserts the pane draws no session list and that picking a chat from Search chats selects it.
• Keeps SessionSidebar.tsx in place for the prototype shell, which still uses it.
• Removing the toggle and scrim clears the pane's last raw-role violations, so App.tsx leaves the ui-rules pending list.
• previewFeatures and quickComposer tests now assert the rail and intra-pane session list are gone.
• New chat, search, and the Recents list move into the window sidebar in a follow-up PR.

Testing

• singleSidebar.test.tsx replaces the overlay test. It fails on the base commit (the pane still draws .acpmux-sidebar) and passes here; it also checks that picking a chat from Search chats selects it. Red and green were run with bun test src/agent-session/acpmux/singleSidebar.test.tsx.
• singleSidebar, searchChatsCommand, NewTabPage and homeLists tests: 18 pass, 0 fail, run locally since PR CI into feat-cmux-next doesn't run webviews tests.
• build-agent-pane-web.sh --check reports the committed bundle is current.