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.

tests/plugin-shape.spec.ts

@@ -52,13 +52,14 @@ describe('dsh-better-sidebar plugin export shape', () => {
   })
 
   it('registers the side card preferences schema with the documented defaults', async () => {
-    const { PrefsSchema, SIDEBAR_PREFS_NS } = await import('../src/config.ts')
+    const { PrefsSchema, SIDEBAR_PREFS_NS, SIDEBAR_PREFS_DEFAULTS } = await import('../src/config.ts')
     expect(SIDEBAR_PREFS_NS).toBe('dsh-better-sidebar')
     const resolved = (PrefsSchema as unknown as {
       (input: Record<string, unknown> | undefined): Record<string, unknown>
     })(undefined)
-    expect(resolved.openByDefault).toBe(false)
-    expect(resolved.defaultWidthPercent).toBe(35)
+    expect(resolved.openByDefault).toBeUndefined()
+    expect(resolved.defaultWidthPercent).toBeUndefined()
+    expect(resolved.changesDiffFloat).toBeUndefined()
     expect(resolved.autoOpenSubagent).toBe(true)
     // A new background job auto-opens the Jobs page too.
     expect(resolved.autoOpenJobs).toBe(true)
@@ -96,7 +97,17 @@ describe('dsh-better-sidebar plugin export shape', () => {
     // default when the stored document predates it.
     const overridden = (PrefsSchema as unknown as {
       (input: Record<string, unknown> | undefined): Record<string, unknown>
-    })({ openByDefault: false, defaultWidthPercent: 45 })
-    expect(overridden).toEqual({ openByDefault: false, defaultWidthPercent: 45, autoOpenSubagent: true, autoOpenJobs: true, agentTerminalTools: false, agentOpenTools: false, bottomPanelAutoTerminal: true, terminalFontFamily: '', terminalFontSize: 13, editorExplorer: false, workspaceFence: true, terminalShell: '', terminalShellArgs: '', titleBarCompat: false, titleBarStripPx: 40, htmlViewerNoSandbox: false, htmlViewerDefaultUnsafe: false, browserNoSandbox: false, browserInterceptLinks: true, browserInterceptHttp: true, browserInterceptHttps: false, browserAllowedLoopback: '', changesDiffFloat: true, tabsEnabled: {}, viewersEnabled: {}, pluginSettings: {} })
+    })({ openByDefault: false, defaultWidthPercent: 45, changesDiffFloat: true })
+    // Schemastery's object schema is OPEN: a document written by an older
+    // plugin version still carrying the retired keys resolves them through
+    // verbatim. They are inert — the typed value the client consumes
+    // (parsePrefs) drops them (tests/prefs.spec.ts) — and the defaults no
+    // longer declare them.
+    // titleBarScheme / titleBarPresetId / customCss are declared WITHOUT a
+    // schema default (the client's parsePrefs supplies them), so they are
+    // absent from a resolved document that never stored them.
+    const { titleBarScheme, titleBarPresetId, customCss, ...schemaDefaults } = SIDEBAR_PREFS_DEFAULTS
+    void titleBarScheme; void titleBarPresetId; void customCss
+    expect(overridden).toEqual({ ...schemaDefaults, openByDefault: false, defaultWidthPercent: 45, changesDiffFloat: true })
   })
 })