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/src/panes/registry.dart

@@ -1,6 +1,6 @@
 /// [PaneRegistry] — backend-side state for all live panes.
 ///
-/// Owns the [NativePty] per pane, generates `p_N` ids, and forwards
+/// Owns the [PtySession] per pane, generates `p_N` ids, and forwards
 /// pty output + lifecycle changes as IPC events via a [DaemonEventSink].
 /// Pane commands (pane.spawn / list / write / resize / close) resolve
 /// against this registry; extension UIs subscribe to the emitted events.
@@ -12,7 +12,7 @@ import 'dart:io' show Platform;
 import 'dart:typed_data';
 
 import '../ipc/envelope.dart';
-import '../pty/native_pty.dart';
+import '../pty/pty_session.dart';
 import 'event_sink.dart';
 import 'pane.dart';
 
@@ -21,7 +21,7 @@ class PaneRegistry {
 
   final DaemonEventSink events;
   final Map<String, Pane> _panes = {};
-  final Map<String, NativePty> _sessions = {};
+  final Map<String, PtySession> _sessions = {};
   final Map<String, StreamSubscription<Uint8List>> _subs = {};
   int _nextId = 1;
 
@@ -55,7 +55,7 @@ class PaneRegistry {
       ...?env,
     };
 
-    final session = NativePty.start(executable: executable, arguments: arguments, columns: cols, rows: rows, workingDirectory: cwd, environment: fullEnv);
+    final session = startPtySession(executable: executable, arguments: arguments, columns: cols, rows: rows, workingDirectory: cwd, environment: fullEnv);
     final pane = Pane(id: id, kind: kind, pid: session.pid, argv: argv, cwd: cwd, title: title);
     _panes[id] = pane;
     _sessions[id] = session;