feat: add beliefs with evidence-derived confidence, contradictions and corrections

Phase 6. Beliefs are propositions with a prior set by epistemic kind and a
confidence computed from typed evidence: sigmoid(logit(prior) + slope ×
(support × diversity − opposition)), where support from a single source
is discounted so repetition cannot manufacture certainty, and bounds keep
the mind away from absolute certainty. Every evaluation stores its
breakdown and appends a version row protected by the append-only
trigger; the belief page shows the arithmetic, evidence for and against,
alternatives, contradictions and history.

Contradiction detection is deterministic first (negation plus token
overlap): a hit emits contradiction_detected, links both beliefs and
makes each counter-evidence for the other so both trend to contested.
Operators can assert beliefs (their statement is the evidence), add
evidence, mark a belief wrong (pinned rejected, history kept) and forbid
an inference; do-not-infer corrections block matching hypotheses.
Retracting a memory retracts the evidence that cited it and re-evaluates
the beliefs concerned.

The associate process proposes low-confidence causal hypotheses from
co-occurring notable observations, and the hypothesis handler turns
admitted ones into beliefs with automatic alternatives, closing demo
scenarios 1 and 2 without any model call.

internal/ui/memory.go

@@ -120,6 +120,11 @@ func (u *UI) memoryRetract(w http.ResponseWriter, r *http.Request) {
 	}
 	u.deps.Events.OperatorAction(r.Context(), m.ID, username(r), "memory.retract", "Operator "+username(r)+" marked a memory inaccurate: "+reason, map[string]any{"memory_id": id, "reason": reason})
 	u.deps.Audit.Log(r.Context(), "memory.retract", "memory", id.String(), map[string]any{"reason": reason})
+	if affected, err := u.deps.Beliefs.RetractEvidence(r.Context(), u.deps.DB, m.ID, "memory", string(id)); err == nil {
+		for _, bid := range affected {
+			u.deps.Beliefs.Reevaluate(r.Context(), u.deps.DB, u.emitF(), m.ID, bid, "supporting memory retracted by operator") //nolint:errcheck
+		}
+	}
 	u.flash(w, r, "ok", "Memory marked inaccurate. It remains in history.", "/minds/"+m.Slug+"/memory/"+id.String())
 }