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/ipc/server.dart

@@ -410,8 +410,12 @@ class IpcServer {
   // -- internals ------------------------------------------------------------
 
   /// `chmod` via `chmod(1)` because dart:io doesn't expose the
-  /// syscall on unix. Cheap; only runs at start/stop.
+  /// syscall on unix. Cheap; only runs at start/stop. No-op on
+  /// Windows: POSIX modes don't exist there, and the socket lives
+  /// under `%LOCALAPPDATA%`, whose per-user ACL already provides the
+  /// user-only gate D-71 wants.
   Future<void> _chmod(String path, int modeBits) async {
+    if (Platform.isWindows) return;
     final octal = modeBits.toRadixString(8).padLeft(3, '0');
     final r = await Process.run('chmod', [octal, path]);
     if (r.exitCode != 0) {