Add editor tooling configuration

clangd fallback flags, clang-format style, clang-tidy checks, and gitignore for the autotools-generated artifacts.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

.clang-tidy

@@ -0,0 +1,19 @@
+# .clang-tidy — nulsl-libc lint configuration.
+#
+# The linter's job is to yell about bugs, not style (style belongs to
+# clang-format). Keep the check list tight and useful.
+
+# A libc owns the reserved namespace by definition (_exit, environ, ...);
+# bugprone-reserved-identifier only makes sense for application code.
+Checks: >
+  clang-analyzer-*,
+  bugprone-*,
+  performance-*,
+  portability-*,
+  -bugprone-easily-swappable-parameters,
+  -bugprone-implicit-widening-of-multiplication-result,
+  -bugprone-reserved-identifier
+
+WarningsAsErrors: ''
+HeaderFilterRegex: 'include/.*'
+FormatStyle: file