feat: ways tune — parallel threshold auto-tuner with discrimination audit

Two modes:
- `ways tune [--apply]`: compute optimal embed_threshold per locale
  entry by scoring against the full corpus. Parallel (cores - 4),
  ~13s for 328 entries on 32 cores.
- `ways tune --audit`: flag entries with low discrimination gap,
  showing which ways they're confused with (confusers).

Two dimensions to optimize:
- Discrimination (gap): description quality — wide gap = precise
- Sensitivity (threshold): auto-computed from discrimination data

The audit surfaces entries where no threshold can give both correct
matches and zero false positives — the description itself needs work.

Author: aaronsb

tools/ways-cli/src/cmd/mod.rs

@@ -20,3 +20,4 @@ pub mod stats;
 pub mod status;
 pub mod suggest;
 pub mod tree;
+pub mod tune;