Add project skeleton: Godot setup, tooling, and smoke test

Pins Godot 4.6.3-stable and GUT 9.6.0 (9.6.1+ reference
AccessibilityServer, unavailable in 4.6.3) via tools/setup.sh. Wires
make check (gdlint + gdformat config for 4-space indent, headless
GUT run, and a JSON well-formedness check ahead of real schema
validation). Seven autoload stubs match the architecture in the
cahier des charges; real logic lands with the following steps.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.editorconfig

@@ -0,0 +1,16 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = space
+indent_size = 4
+
+[*.{gd,py,json,md,cfg,yml,yaml,sh}]
+indent_style = space
+indent_size = 4
+
+[Makefile]
+indent_style = tab