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

@@ -1,4 +1,5 @@
 build/
+local/
 .cache/
 
 # Atmel Studio: generated per machine, its build outputs, and per-user state