refactor(shell): retire the plugin's own right panel and free windows

DSH 0.1.5 owns the right column (its native Sidebar hosts every tab type
this plugin registers), so the plugin's own right panel, its width drag /
new-session seed prefs, and the free-window feature are gone. The bottom
workbench is the only plugin-owned surface left, and its expand/collapse
control is registered into DSH's session-header utilities list slot (the
header's corner seat belongs to the native sidebar's own control).

- state: one `bottomSplits` tree; `makeDefaultState()` takes no arguments;
  `toggleBottomPanel` / `openTabInBottomPane` are the only landing
  reducers; `setBottomHeight` and `sanitizeState` share the
  `CONVERSATION_MIN` cap (a persisted tall height no longer snaps on the
  first drag).
- service: every landing in the workbench (creation or focus) expands it;
  `SIDEBAR_FEATURES` drops `'floatWindows'`; the open/close/activate tab
  pools cover the single tree.
- shell: `Sidebar.tsx` renders only the bottom workbench; the toggle lives
  in `sidebar/bottom-toggle.tsx`; layout-push pushes height only (no width,
  no `--dsh-sidebar-width`).
- prefs: `openByDefault` / `defaultWidthPercent` / `changesDiffFloat`
  removed from the schema, defaults, client parser, settings page, and all
  20 dictionaries; the retired keys in stored documents are ignored.
- tests: free-window specs deleted, panel/float assertions dropped,
  remaining suites adapted (124 files / 1287 tests); the mount lane asserts
  the header toggle and sweeps the native sidebar's guide page.
- docs: README / README_EN / guide §0,§11 / AGENTS test map.

src/client/changes/ChangesTab.tsx

@@ -3,9 +3,9 @@
  * (repository truth: staged/unstaged files, commit box, history) and the
  * session round (agent truth: every file the model read, wrote, or edited).
  * Both lenses preview their selections in a shared resizable bottom pane
- * ({@link DiffPane}); git targets expand into the dedicated diff tab —
- * docked in a pane, or floated as a free window per the tab's setting.
- * The active lens and the pane height persist in the tab's meta, so the tab
+ * ({@link DiffPane}); git targets expand into the dedicated diff tab docked
+ * in the workbench's diff pane. The active lens and the pane height persist
+ * in the tab's meta, so the tab
  * reopens exactly where it was left.
  *
  * The session events ride the host's `changes.ops` route (the client
@@ -19,7 +19,7 @@ import type { TabComponentProps } from '../service.ts'
 import { t } from '../locales.ts'
 import { api } from '../api.ts'
 import { usePolling } from '../use-polling.ts'
-import { floatTab, type SidebarDiffRef } from '../state.ts'
+import type { SidebarDiffRef } from '../state.ts'
 import { GitLens } from './GitLens.tsx'
 import { SessionLens } from './SessionLens.tsx'
 import { DiffPane, diffTabOf, type ChangesPreview } from './DiffPane.tsx'
@@ -137,18 +137,11 @@ export function ChangesTab({ ctx, store, scope, tab, visible, onOpenFile, onOpen
     setPreview({ kind: 'op', path, op, prior: knownContentBefore(ops, path, op) })
   }
 
-  /** Expand the current git preview into the dedicated diff tab: docked
-   *  into the shell's diff pane, or floated as a free window centered on
-   *  the viewport when the tab's diff-open setting asks for it (default). */
+  /** Expand the current git preview into the dedicated diff tab, docked
+   *  into the workbench's diff pane. */
   const expandPreview = (): void => {
     if (preview?.kind !== 'git') return
-    const diffTab = diffTabOf(preview.ref)
-    onOpenDiff?.(diffTab)
-    if (store.getPrefs().changesDiffFloat !== false) {
-      const x = Math.round(window.innerWidth / 2)
-      const y = Math.round(window.innerHeight / 2)
-      store.reduce(state => floatTab(state, diffTab.id, x, y))
-    }
+    onOpenDiff?.(diffTabOf(preview.ref))
   }
 
   const previewKey = (target: ChangesPreview): string => target.kind === 'git'