#24564 cmux NEXT: make Session Info chrome content-first

This PR refines cmux NEXT's Session Info side panel around Leo's two observations:

"also this shouldn't be TITLED session info..."

"also this transition between window sizes makes things extremely 'off' like from that transition point to the point where there's just barely enogh space to fill both the main agent chat and the session info summary, that range, just have a smooth and fast animation to ever so slightly bump the agent thing a little to the left so that it keeps centering as the session info is there pushing it in a bit..."

Before, the panel repeated “Session Info” in a title row and the centered chat stayed put while the floating panel approached the point where it had to push the chat. After, the shipped chrome removes that redundant title row and keeps a floating close control with the content; the gallery preserves the titled and compact close alternatives for comparison. During the final 64px of the overlay range, the chat stage eases left toward the exact room the pushed layout will take, then the existing per-frame width follow takes over without a jump.

The chosen chrome follows cmux-design-references/apps/cmux/live/NOTES.md and the live cmux panel references: a small rounded glass surface with a quiet close control and content-first spacing. ChatInspector.gallery.tsx includes three chrome versions (chrome-close-only chosen, chrome-title, chrome-compact-close) and the motion gallery includes resize-handoff.

The handoff animation has an explicit stop condition: it ends when the measured pane reaches the push boundary, where the shift equals the pushed panel's half-room and the push marker disables the transition for that frame. Cleanup clears --acpmux-chat-shift and data-inspector-chat-placement when the panel is not drawn or unmounts; Reduce Transparency/Reduce Motion uses the existing no-motion path.

Validation: targeted inspector, surface, reveal, and header tests pass; motion-scale, layers, prepush, formatting, catalog, and protocol checks pass. The repository type gate still reports four pre-existing errors in unrelated minimap/new-tab tests.

Agent session: cmux16

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Summary by cubic

Refines cmux NEXT's Session Info panel: removes the redundant "Session Info" title row in favor of a floating close control, and smooths the resize between floating and pushed layouts so the chat no longer jumps at the boundary.

• The shipped chrome is close-only; the gallery keeps the titled and compact close alternatives for comparison.
• A new resize-handoff gallery shows the chat easing left during the final 64px of the overlay range so the pushed layout lands on the same center.