Add .editorconfig (closes #27) (#28)

Adds `.editorconfig`, copied verbatim from `sneak/dnswatcher` (`root = true`; space/4/lf/utf-8/trim/final-newline, with `[Makefile]` overriding to tab). secret had none. Resolves #27.

`make check` is green on the host (`0 issues`, all packages pass). Note: `docker build .` fails only on a pre-existing, environmental `memguard` `mlock` / `RLIMIT_MEMLOCK` issue in the 10MB-secret test — it reproduces identically on pristine `origin/main` with this file removed, and secret's real CI sets the memlock ulimit for it. A static config file cannot affect Go tests, so this change is clean; the container ulimit is a separate pre-existing concern.

Left open for review (not merged).

Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #28
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

.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