#25638 Agent chat: header tools sit on the tab strip, with no box or line behind them

The agent chat header tools now share the native tab strip surface without a page-painted box or seam behind them. Leo’s words were: “the session rail session info thing, like, you literally didn't do. I mean, why is the button not toggled? Why is the button area still, like, just garbage? What is this? There's still that line of that box with the background that just always is there, and, like, you still haven't done the full-length thing.” This PR covers the “button area … garbage” and “line of that box” portion only.

The box came from the agent page root and .acpmux-shell painting --agent-page-bg across the offered tab row; the line came from .acpmux-header[data-tab-row]::after’s 12px gradient. Native pane framing is disabled for agent chats, so the native pane border/corner was not the source. The fix marks the shell while the row is offered, makes the document root transparent for that state, paints the page background below the row, and removes the seam pseudo-element. Tool positions, hit targets, tabs, and other pane surfaces are unchanged.

Before / after

Fleet capture evidence will be added after the Mac build for this exact head.

Risk

Low. The transparency selector is scoped to an agent shell with an active tab-row offer, and the existing page background resumes below the row. The focused regression checks the shell marker, scoped surface rule, and removed seam.

Validation

• bun test webviews/test/chat-header-tab-row-surface.test.ts webviews/test/chat-header-plain-icons.test.ts (pass)
• bun run --cwd webviews check:fixed-waits (pass)
• bun run --cwd webviews check:design-tokens (pass; baseline ratchet reduced by removing the deleted 12px seam)
• scripts/cmux-next/prepush-gate.py (pass)
• check:layers remains red on the existing webviews/src/pages/keybindings/styles.css backdrop-filter baseline violation; typecheck/lint cannot complete because this checkout lacks generated locale files.

agent-identity footer

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