dsh-rightbar-exit-strip: Add two exit entry points for the fullscreen right sidebar.

When the DSH right sidebar is in fullscreen, it covers the viewport, leaving only the top-right corner panel controls as the exit option. Maintained by 2315137135, categorized as client-side, and requiring no build process, dsh-rightbar-exit-strip is pure JavaScript. It adds a 5px semi-transparent exit strip to the left edge of the fullscreen state (expanding to 11px on hover, displaying a right-pointing arrow, and changing to the brand color; keyboard focus also triggers expansion, with Enter to exit), and supports double-clicking the divider between the session and the right sidebar to directly enter fullscreen. Both entering and exiting fullscreen are handled via `ctx.sidebarRight.commandTarget()` with `toggleFullscreen()`, and are recorded in the layout history. Installation is done via `plugin_manager install_bundle` with the target `github:2315137135/dsh-rightbar-exit-strip`; `git+https` is also supported. Refresh the page after installation. Implementation-wise, it only renders when in fullscreen mode, detecting `[data-sidebar-right-panel="fullscreen"]` and positioning based on `getBoundingClientRect()`. It also utilizes `ResizeObserver`, `resize`, and `MutationObserver`. The vertical strip is portaled to `document.body` to avoid the stacking context of `shell.overlay`. For double-clicking, it uses the capture phase to count `pointerdown` events (twice within 500ms and 6px range), as the divider's `preventDefault` on `pointerdown` suppresses the `dblclick` event. The plugin depends on framework markers such as `[data-side]=rightbar` and `[data-sidebar-right-mode]`, which need to be confirmed after a major version upgrade of Harness. License MIT, version 0.1.0.