Correct the documented local harness constructor
Problem
The README still names an ephemeral filesystem::LocalHarness::new constructor that the crate does not provide.
Design
Correct only that constructor sentence: LocalHarness::open(LocalHarnessConfig::new(path, (model, provider))).await, with filesystem-local,native-execution, constructs the durable local host and stock model loop for a normal checkout. #495 is now merged; this describes existing main code.
Deletions
Remove the nonexistent ephemeral-constructor claim. README-only delta: +1/-1, net 0.
Decisions
Keep the correction factual and confined to the existing paragraph. Runtime recovery fixes remain in #672. Human README approval remains unchecked.
Tests run
git diff --check; verified constructor signature and feature gates against main. Confirmed that the PR changes only rust/crates/harness/README.md. No executable code changed.
- README text written by a human