chore: machine-local state has a home here too

Guidance rule 15 puts every machine-local artefact under a gitignored local/,
and this repo could only have committed one.

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

.gitignore

@@ -12,6 +12,7 @@ Debug
 
 # CMake / clangd
 /build/
+/local/
 compile_commands.json
 .cache/