#17348 fix(agent pane): use the main sidebar for agent chats

The agent pane currently renders a second icon rail and Pinned/Projects session list beside the main cmux sidebar. This makes the same agent chats appear in two navigation surfaces.

This change keeps the existing pane session toggle but starts the optional inner list closed at every width. It stays closed across width changes and after narrow-pane dismissal, while the main sidebar continues to expose agent chat tabs through its existing agentChat rows. New Agent Chat and Search Agent Chats remain available through the existing palette actions.

Validation:

• git diff --check
• Focused overlay regression expectations updated for the collapsed default and explicit toggle behavior
• Native build and runtime capture will run through the fleet, per oom-note.md

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Summary by cubic

Stops the agent pane's optional session list from appearing next to the main cmux sidebar, so agent chats show in only one navigation surface. The inner list now starts collapsed at every pane width, stays closed across width changes and after narrow-pane dismissal, and opens only when the user explicitly toggles it; agent chats remain available through the main sidebar's agentChat rows and the palette actions. Regenerates the pane bundle for the collapsed default.

• Updates the focused overlay regression expectations for the collapsed default and explicit toggle behavior.