#26499 PR3: remove top-right header icon plates

PR3: every top-right header control is a plain icon with no background plate. The shipped chat header already has the compact SF Symbol group from #26377; this closes the remaining plate path in the floating chat header and keeps hover/pressed emphasis on the individual control's ink, never on the group.

Leo: “top-right icons have NO background plates at all — plain icons; highlight only on hover/press of that one control.”

The private 2026-10-28 reference 1-topright-toolbar.png is not attached. It showed filled plates behind the list, Changes, Terminal, Browser and overflow controls. The implementation keeps the 24px hit targets and removes the floating header/bar background fills for rest, hover, popup-open and pressed states. The group remains transparent. Existing menu surfaces retain their own menu highlight.

Base commit 702344519b8 removed the native new-terminal and new-browser resting buttons from every pane; PaneToolbar on the current nightly base still has only the pane + and … creation paths. This PR does not reintroduce those buttons.

Gallery: agent-pane.header-tools/plain-icons is the before/after gallery treatment. The selected treatment is the plain glyph group: it matches the Claude/Codex reference and makes the control state readable through the one symbol that owns it. Private captures remain local and are not attached.

Validation: the browser regression covers the normal tab-row shell and the floating header, asserting transparent background, zero border, and no shadow on a pressed floating tool. The full web check and pre-push gate are required for the final head. No new timer, observer, animation loop, or transcript-row work is added.

Risk: floating chat controls lose only decorative fills; their hit targets, focus ring, menu behavior, popup-open state, and pressed ink remain. Native pane creation actions remain base behavior for every pane kind.

Before/after captures will be added from the current base and this exact head using the gallery capture route; private Leo references are not published.

Agent session: agent session

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Summary by cubic

Removes the background plates behind the floating chat header's top-right controls so they appear as plain icons in every state. Active controls read through their own ink, the group never gains a backdrop, and disabled tools render muted.

• Hit targets, menu behavior, and pressed ink are unchanged; only decorative fills are removed.
• A focus-visible outline is now enforced on header tools in the floating, floating-bar, and inline hosts, overriding shared pane styles that would suppress the ring.
• The regression test loads the shared pane styles and verifies transparent background, no border/shadow, and correct ink across rest, hover, pressed, popup-open, focus-visible, and disabled for all three header hosts.
• The test parser strips !important before judging fills, so the focus rule's background:none !important is counted as no fill.