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/builtin/terminal/src/terminal_pane.dart

@@ -66,18 +66,16 @@ class _TerminalPaneState extends State<TerminalPane> {
       return;
     }
 
-    final shell = Platform.environment['SHELL'] ?? '/bin/bash';
+    // Windows has no $SHELL convention and no login-shell flag —
+    // PowerShell 7 first, classic PowerShell as the always-there
+    // fallback.
+    final shell = Platform.isWindows ? null : (Platform.environment['SHELL'] ?? '/bin/bash');
+    final argv = shell != null ? [shell, '-l'] : ['powershell.exe', '-NoLogo'];
     final cwd = Directory.current.path;
 
     final response = await ipc.request(
       'pane.spawn',
-      args: {
-        'argv': [shell, '-l'],
-        'kind': PaneKind.terminal.wire,
-        'cwd': cwd,
-        'cols': _terminal.viewWidth,
-        'rows': _terminal.viewHeight,
-      },
+      args: {'argv': argv, 'kind': PaneKind.terminal.wire, 'cwd': cwd, 'cols': _terminal.viewWidth, 'rows': _terminal.viewHeight},
     );
     if (!mounted) return;
     if (!response.ok) {