feat(ml): stricter auto-apply defaults to cut misfires (milestone 139)

head_auto_apply_precision 0.97→0.98, head_auto_apply_min_positives 30→50,
ccip_auto_apply_threshold 0.92→0.95 (operator-asked). Model defaults change for
fresh installs; migration 0081 bumps the existing singleton row IFF still at the
old default (won't clobber a deliberate operator change). ml_admin bounds already
permit these. Fixed a stale comment in the auto-apply test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_head_auto_apply.py

@@ -88,7 +88,7 @@ def test_sweep_dry_run_counts_but_writes_nothing(db_sync):
 
 
 def test_sweep_skips_under_supported_head(db_sync):
-    # n_pos below head_auto_apply_min_positives (default 30) → a precise-looking
+    # n_pos below head_auto_apply_min_positives (default 50) → a precise-looking
     # but under-supported head never fires.
     img = _img(db_sync, "c" * 64, _emb(0))
     tag = Tag(name="weaktag", kind=TagKind.general)