port clide runtime to Windows (ConPTY, AF_UNIX, PATHEXT)

Bring the runtime up on Windows without disturbing the POSIX paths.

PTY: introduce a platform-neutral PtySession contract with a factory
that picks NativePty (posix_openpt/posix_spawn) or the new WindowsPty
(ConPTY via CreatePseudoConsole). The pane registry programs against
the interface; NativePty now implements it.

IPC: the per-workspace AF_UNIX socket lives under %LOCALAPPDATA% and
is hashed from a canonical workspace key (backslash + ASCII-folded
case) so the Dart server and the C client agree despite NTFS case-
insensitivity. The C client grows a Win32 shim (winsock afunix);
chmod is a no-op on Windows where the per-user ACL is the gate.

Toolchain: PATH probing splits on ';' and tries PATHEXT extensions;
the shell defaults to PowerShell (pwsh, then powershell); tmux is
treated as optional since it has no Windows build; dugite falls back
to PATH git for now.

Build: add `make build-windows`, a clide-cli MSVC build wrapped by
ci/build_cli_windows.sh, and a ConPTY smoke-test suite that self-
skips off-platform.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/kernel/src/tool_check.dart

@@ -19,7 +19,9 @@ class ToolCheck extends ChangeNotifier {
 
   Future<void> check() async {
     pqlOk = _existsOnPath('pql');
-    tmuxOk = _existsOnPath('tmux');
+    // tmux has no Windows build; absence there is the documented
+    // no-tmux mode, not a failed check.
+    tmuxOk = Platform.isWindows || _existsOnPath('tmux');
     gitOk = _existsOnPath('git');
     checked = true;
     notifyListeners();
@@ -29,9 +31,16 @@ class ToolCheck extends ChangeNotifier {
   /// Uses direct file-existence checks — works inside a macOS sandbox
   /// without needing to exec `which`.
   static bool _existsOnPath(String name) {
-    for (final dir in expandedPath.split(':')) {
+    final sep = Platform.isWindows ? ';' : ':';
+    for (final dir in expandedPath.split(sep)) {
       if (dir.isEmpty) continue;
-      if (File('$dir/$name').existsSync()) return true;
+      if (Platform.isWindows) {
+        for (final ext in const ['.exe', '.bat', '.cmd', '.com', '']) {
+          if (File('$dir\\$name$ext').existsSync()) return true;
+        }
+      } else {
+        if (File('$dir/$name').existsSync()) return true;
+      }
     }
     return false;
   }