chore: add dependabot config and editorconfig

Add .github/dependabot.yml to keep GitHub Actions and Cargo dependencies
current via weekly update PRs (the proactive half of dependency hygiene
alongside the security-audit workflow).

Add .editorconfig for repo-wide whitespace consistency (LF, final
newline, trimmed trailing whitespace, UTF-8) and 2-space YAML/TOML/
Markdown. Rust formatting is left to rustfmt, so there's no .rs rule.

Author: mwln

.editorconfig

@@ -0,0 +1,13 @@
+# Rust formatting is owned by rustfmt; this only covers repo-wide whitespace
+# rules and the non-Rust files rustfmt doesn't touch.
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+charset = utf-8
+
+[*.{yml,yaml,toml,md}]
+indent_style = space
+indent_size = 2

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: weekly
+  - package-ecosystem: cargo
+    directory: "/"
+    schedule:
+      interval: weekly