chore(release): consolidate v3.12.3 release notes, changelog and test fixes

Merge previously unreleased v3.12.4 content into v3.12.3:
- CHANGELOG: combine [Unreleased] into [3.12.3], clear [Unreleased]
- Release notes (zh/en/ja): add Copilot proxy, macOS signing,
  Reasoning Effort, OpenCode SQLite, Codex 1M toggle, Disable
  Auto-Upgrade toggle, and contributor thanks
- Fix test mocks for skill backup/restore hooks
- Fix schema migration test missing providers table
- Fix TempHome to save/restore CC_SWITCH_TEST_HOME env var

tests/components/UnifiedSkillsPanel.test.tsx

@@ -11,6 +11,8 @@ const toggleSkillAppMock = vi.fn();
 const uninstallSkillMock = vi.fn();
 const importSkillsMock = vi.fn();
 const installFromZipMock = vi.fn();
+const deleteSkillBackupMock = vi.fn();
+const restoreSkillBackupMock = vi.fn();
 
 vi.mock("sonner", () => ({
   toast: {
@@ -25,9 +27,22 @@ vi.mock("@/hooks/useSkills", () => ({
     data: [],
     isLoading: false,
   }),
+  useSkillBackups: () => ({
+    data: [],
+    refetch: vi.fn(),
+    isFetching: false,
+  }),
+  useDeleteSkillBackup: () => ({
+    mutateAsync: deleteSkillBackupMock,
+    isPending: false,
+  }),
   useToggleSkillApp: () => ({
     mutateAsync: toggleSkillAppMock,
   }),
+  useRestoreSkillBackup: () => ({
+    mutateAsync: restoreSkillBackupMock,
+    isPending: false,
+  }),
   useUninstallSkill: () => ({
     mutateAsync: uninstallSkillMock,
   }),
@@ -68,6 +83,8 @@ describe("UnifiedSkillsPanel", () => {
     uninstallSkillMock.mockReset();
     importSkillsMock.mockReset();
     installFromZipMock.mockReset();
+    deleteSkillBackupMock.mockReset();
+    restoreSkillBackupMock.mockReset();
   });
 
   it("opens the import dialog without crashing when app toggles render", async () => {