Doc: Keep git to long-term records and ignore local scratch

Ignore /local for short-term benchmark evidence, one-off A/B raw logs, and ad-hoc probes, and record the repository hygiene rule in AGENTS.md.

.gitignore

@@ -1,5 +1,6 @@
 /build
 /output
+/local
 /scripts/__pycache__
 assets/2mass/processed/all_sky/*.csv
 assets/2mass/processed/all_sky/.done/