feat(diff): add branch scope to context panel diff view

Show every change on the current branch relative to its base in the
Changed/Staged/Last turn dropdown. The base comes from the branch's
reflog record or an explicit per-branch user choice (persisted), never
a main/master guess; when git has no record the user picks a base once
from a searchable branch list.

- server: GET /api/git/branch-base (reflog-derived base),
  GET /api/git/range-files (name-status -z with rename/copy
  destination paths and -C copy detection)
- shared UI: optional getBranchBase/getGitRangeFiles runtime APIs
  with boundary parsing; persisted per-branch overrides keyed by
  runtime+directory+branch
- DiffView: branch scope with confirmed-unavailability coercion of
  persisted tabs (detached HEAD, default-branch checkout, metadata
  settled without a default), range-invalidated diff cache guarded
  against stale completions, bounded branch-metadata retry, read-only
  diff actions in branch scope; hidden in VS Code
- helper module branchDiffScope.ts with tests for coercion,
  availability, race conditions, and retry exhaustion

packages/ui/src/stores/useUIStore.ts

@@ -20,7 +20,7 @@ import { isVSCodeRuntime } from '@/lib/desktop';
 export type WorkspaceSurface = 'chat' | 'plan' | 'git' | 'diff' | 'terminal' | 'files' | 'context' | 'diagram';
 /** @deprecated Use WorkspaceSurface. */
 export type MainTab = WorkspaceSurface;
-export type PendingDiffScope = 'working' | 'staged' | 'turn';
+export type PendingDiffScope = 'working' | 'staged' | 'turn' | 'branch';
 export type ContextPanelMode = 'diff' | 'walkthrough' | 'file' | 'context' | 'plan' | 'chat' | 'browser' | 'git' | 'pr' | 'notes' | 'terminal';
 export type MermaidRenderingMode = 'svg' | 'ascii';
 export type UserMessageRenderingMode = 'markdown' | 'plain';
@@ -205,7 +205,7 @@ const normalizeContextTabLabel = (value: string | null | undefined): string | nu
 };
 
 const normalizePendingDiffScope = (value: unknown): PendingDiffScope | null => {
-  return value === 'working' || value === 'staged' || value === 'turn' ? value : null;
+  return value === 'working' || value === 'staged' || value === 'turn' || value === 'branch' ? value : null;
 };
 
 const buildDefaultContextPanelTabDedupeKey = (mode: ContextPanelMode, targetPath: string | null): string => {