add macOS desktop target with sandbox entitlements

OS-detecting Makefile (make run works on macOS/Linux/Windows), 1280x720
default window in MainMenu.xib, squared app icons, sandbox with scoped
SBPL exceptions for subprocess execution, _DARWIN_C_SOURCE for ptyc
compilation, expanded PATH merging Homebrew and ~/.local/bin for GUI
apps, native traffic dots skipped on macOS (titlebar owns them).

DebugProfile.entitlements is gitignored (machine-specific SBPL paths);
a template with __HOMEDIR__/__PROJECTS__/__SHELL__ placeholders is
committed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ptyc/ptyc.c

@@ -18,6 +18,9 @@
 
 #define _POSIX_C_SOURCE 200809L
 #define _XOPEN_SOURCE 600
+#ifdef __APPLE__
+#define _DARWIN_C_SOURCE  /* CMSG_SPACE / CMSG_LEN on macOS */
+#endif
 
 #include <ctype.h>
 #include <errno.h>