refactor: remove the tmux overlay experiment

[arzafran]

What this does

Removes what was left of the tmux overlay experiment. It could only be turned on with a hidden defaults write key, and no Settings UI reached it. The default behavior does not change: attention flashes still run on the surface itself, and the unread ring still shows.

About 1,000 lines go, roughly a third of them tests.

Review order

1. Sources/Panels/TerminalPanel.swift: triggerFlash keeps only the .surface path.
2. Sources/Workspace.swift, Workspace+Bonsplit.swift, Workspace+Surfaces.swift: these remove the overlay-only published fields and the configureTerminalPanel hook. didChangeGeometry still schedules terminal geometry and focus reconcile. It no longer republishes a layout snapshot on every resize.
3. Sources/WorkspaceContentView.swift, WindowOverlayControllers.swift, ContentView.swift: these remove the overlay view, model, window controller and rect helpers.
4. Tests: removes the tests that only covered the .bonsplitPane target. Mixed tests keep their unread and focus assertions. TmuxWorkspacePaneOverlayTests is renamed AppIconAndFlashStyleTests for the tests that remain.
5. docs/removed/tmux-active-pane-overlay.md now covers the whole experiment and has a restore line.

Test plan

• CI is green, including unit tests and UI regressions
• In a build, an agent notification flashes the terminal surface
• Resizing a split pane still resizes the terminal and keeps focus