Integrate G15 submodules with Debian 13 packaging and devcontainer

.gitmodules

@@ -0,0 +1,15 @@
+[submodule "libg15"]
+	path = libg15
+	url = https://gt.uo1.net/mikhail/libg15
+[submodule "g15daemon"]
+	path = g15daemon
+	url = https://gt.uo1.net/mikhail/g15daemon
+[submodule "g15macro"]
+	path = g15macro
+	url = https://gt.uo1.net/mikhail/g15macro
+[submodule "libg15render"]
+	path = libg15render
+	url = https://gt.uo1.net/mikhail/libg15render
+[submodule "g15stats"]
+	path = g15stats
+	url = https://gt.uo1.net/mikhail/g15stats