docs: record decision materiality rule

docs/current-handoff.md

@@ -2671,3 +2671,7 @@ Tested whether the Confidence Engine can independently recognise when quantified
 
 **Diagnosis:** Classification C — NO SUFFICIENCY RULE + CONTINUATION BIAS
 **Missing distinction:** MATERIALITY / DECISION-RELEVANCE RULE
+
+---
+
+Experiment 60B.4 implemented the minimal correction: added a domain-general decision-sufficiency rule to the prompt (not schema, not validator). The rule instructs: uncertainty alone is not sufficient reason to continue; continuation requires a specific material factor that could change the preferred option; when no such factor remains, resolve the existing decision context rather than asking a generic question. Prompt-level addition only — three sentences in a new "Decision Sufficiency Rule" section in prompt-builder.js. Deterministic tests confirm all five positive and six negative criteria pass (86/86). No live regression performed. Status pending Rob's review.

docs/experiment-60b4.md

@@ -0,0 +1,68 @@
+# Experiment 60B.4 — Decision Materiality Rule (prompt-only)
+
+**Branch:** `feature/decision-sufficiency-v0.26`  
+**Date:** 2026-08-13  
+**Status:** Complete  
+**Type:** PROMPT-ONLY — Bounded instruction addition plus deterministic prompt tests. No live model calls.
+
+## Reasoning gap from 60B.3
+
+60B.3 confirmed the engine has no independent decision-sufficiency rule. Rule 20 states:
+
+> "Return selectedQuestion as null only when no consequential unresolved unknown remains."
+
+This defines *when* to return null but does not define what makes an unknown non-consequential. The term "consequential" is undefined at decision level. Live results (60B.1 vs 60B.2) show the model defaults to generic continuation when no explicit closing cue exists — even when option evidence is quantified on both sides.
+
+The gap: **uncertainty remains** is always true during investigation. The prompt does not instruct the model to distinguish this from **remaining uncertainty could materially change which option is preferred**.
+
+## New prompt rule
+
+Added section "Decision Sufficiency Rule" between Proposal Rules and Decision Option Structure Rules in `lib/graph/prompt-builder.js`:
+
+> An unresolved decision between options should not remain open merely because some uncertainty still exists.
+>
+> Keep a decision context unresolved only when you can identify a specific unresolved factor that could materially change which option is preferred.
+>
+> If the currently supported evidence is sufficient to distinguish the options and no such material unresolved factor remains, resolve the existing decision context and do not ask a generic continuation question.
+
+## Why this is domain-general
+
+- No financial vocabulary (no £, $, payback, cost comparison)
+- No relocation or industry-specific terms
+- No numeric thresholds or calculation frameworks
+- No keyword-based routing or taxonomic classification
+- The three semantics apply to any decision between options with competing evidence:
+  1. uncertainty alone is not sufficient reason to continue
+  2. continuation requires a specific material factor that could change the preferred option
+  3. when no such factor remains, resolve the existing decision context rather than asking a generic question
+
+## Focused test results
+
+**Command:** `npx vitest run tests/graph/prompt-builder.test.js`  
+**Result:** PASS (86/86)
+
+New materiality tests verify:
+- Positive: uncertainty-alone-is-not-enough, specific-material-factor-required, could-change-preferred-criterion, resolve-when-no-material-factor, generic-continuation-discouraged, Rule-20-preserved, domain-generality
+- Negative: no financial thresholds, no currency examples, no relocation examples, no automatic-resolution-when-better-looking, no new schema fields, no new node kinds
+
+## What remains unproven until live regression
+
+1. **Stability** — deterministic prompt tests confirm the instruction text is present and well-formed, but do not verify the model follows it consistently across repeated runs
+2. **Cross-domain generalisation** — single-prompt-test coverage does not prove the rule works outside the test cases' structural patterns
+3. **Interaction with existing rules** — no regression test confirms the materiality rule does not interfere with Rule 20, option structure rules, or the semantic-to-mutation contract
+4. **Edge cases** — decisions where multiple partially-material factors exist; decisions with equal evidence across options; ambiguous factor specificity
+
+## Production changes
+
+| File | Change |
+|------|--------|
+| `lib/graph/prompt-builder.js` | Added "Decision Sufficiency Rule" section (3 sentences, 3 semantics) |
+| No schema changes | |
+| No validator changes | |
+| No question-selection code changes | |
+| No provider integration changes | |
+
+## Commit messages
+
+Production/tests: `feat(reasoning): add decision materiality rule`  
+Documentation: `docs: record decision materiality rule`