Merge main into windows-support

Brings windows-support up to date with main (T-404/405/406, T-413–416,
T-421, the T-422 workspace-lifecycle epic, and the 2.4.0 release).

Conflict resolutions:
- terminal_pane.dart: keep the Windows PowerShell shell selection and
  main's workspace-cwd fix (T-381) together.
- tool_check.dart: accept main's deletion (dead, unreferenced code).
- CHANGELOG.md: keep both Unreleased sections.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pubspec.lock

@@ -338,14 +338,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.0.0"
-  mocktail:
-    dependency: "direct dev"
-    description:
-      name: mocktail
-      sha256: "5e1bf53cc7baa8062a33b84424deb61513858ea05c601b8509e683815b5914aa"
-      url: "https://pub.dev"
-    source: hosted
-    version: "1.0.5"
   node_preamble:
     dependency: transitive
     description: