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/pty/pty.dart

@@ -1,8 +1,10 @@
-/// PTY subsystem — spawn child processes under a PTY via posix_openpt()
-/// + posix_spawn(), expose their master fd as a byte stream. Desktop
-/// IDE's pane model (terminal / Claude / future tmux wrappers) rides on
-/// this.
+/// PTY subsystem — spawn child processes under a PTY and expose their
+/// output as a byte stream. POSIX uses posix_openpt() + posix_spawn();
+/// Windows uses ConPTY. Desktop IDE's pane model (terminal / Claude /
+/// future tmux wrappers) rides on this.
 library;
 
 export 'env.dart' show clidePtyEnvDefaults, mergePtyEnv;
 export 'native_pty.dart' show NativePty;
+export 'pty_session.dart' show PtySession, startPtySession;
+export 'windows_pty.dart' show WindowsPty;