Add .editorconfig and Gitea CI workflow (closes #14)

Copy .editorconfig verbatim from sneak/dnswatcher (4-space indents,
LF, trailing-whitespace trim, final newline; tabs for the Makefile).

Add .gitea/workflows/check.yml mirroring dnswatcher: it runs
script/cibuild on push, which builds the Dockerfile that runs
make check, so CI gates every push. The checkout action is pinned by
commit SHA as that repo pins it. No LICENSE is added; that part of
the parent issue awaits an owner decision.

Model: opus-4-8

.editorconfig

@@ -0,0 +1,12 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[Makefile]
+indent_style = tab