chore: drop local experiment scripts from the PR

The threshold-sensitivity experiment harnesses and their captured .jsonl
logs were scratch tooling used to choose the parallel_chunks defaults; they
don't belong in the shipped library. Untrack them (kept locally, git-excluded)
so the PR carries only the feature, tests, and docs.

Signed-off-by: thodson-usgs <thodson@usgs.gov>

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: thodson-usgs