feat: raise the default irreversible hold threshold to 0.9

[DevMortimer]

The action guard held every call the judge scored at or above 0.7 for irreversible. On the recorded action-guard corpus, those judge-only holds were false alarms: the judge's error rate is 15–42% below confidence 0.8 and under 1% above it.

Change

• Default action.irreversible.confirm (the hold threshold) is 0.9 instead of 0.7. Calls the judge scores 0.5 to 0.9 warn instead of holding.
• warn stays 0.5. Pattern holds (destructive commands, deny rules) and user or project overrides are unchanged.
• Restore the old behaviour with "action": { "irreversible": { "confirm": 0.7 } }.
• Docs: the confidence-band table, the held-out split, the limits of the evidence, and the method source (Li, Miao, Krishnan, Padman, "JEV-as-a-Judge: Accept When Confident, Escalate When Unsure", arXiv:2609.26550).
• Version bump to 0.75.0.

Evidence

confidence	items	error	false alarms	misses
0.50–0.60	118	42.4%	49	1
0.60–0.80	493	15.2%	72	3
0.80–0.95	4,937	0.8%	28	13
0.95–1.00	9,798	0.1%	0	8

A 0.9 cutoff chosen on a random half of the corpus and checked on the other half removed about 52 false alarms and lost no true catch. The offline calibration report: holds 48 → 25 (all 23 judge-only holds become warnings), regretted calls held 0 → 0. Limits: only 26 regretted calls, so this is a direction rather than a fitted threshold; the remaining misses sit at high confidence, where no threshold reaches them.

Verification

• npm run check: 1207 pass / 0 fail before and after; typecheck and build clean.