experiment(15): specify facilitator behaviour — behavioural model for Phase 5

- Create docs/facilitator-behaviour.md: behavioural specification of the
  Confidence Engine with 14 identified behaviours (Orient, Acknowledge,
  Observe pattern, Clarify, Validate, Connect, Challenge assumption, Refine
  understanding, Expose uncertainty, Decide direction, Know when to pause,
  Avoid premature closure, Communicate confidence honestly, Progressively
  narrow focus).

- Update docs/design-evolution-log.md: add Experiment 15 entry documenting
  what Experiment 14 proved, what emerged (the gap is behavioural not visual),
  and why the next phase focuses on conversation behaviour over UI.

- Update .claude/ux-guidelines.md: add Facilitator Behaviour section with
  core behavioural principles, anti-patterns, state-aware selection criteria,
  and architecture relationship.

No code changes — this is a behavioural specification for future implementation.

.claude/ux-guidelines.md

@@ -533,3 +533,69 @@ Principles:
 - Narrative should evolve as evidence changes.
 - Narrative must never invent facts absent from the graph.
 - Narrative explains uncertainty rather than exposing graph mechanics.
+
+## Facilitator Behaviour
+
+The facilitator is defined by patterns of action, not by its words.
+The same investigation state can produce different behaviours depending on context and history.
+
+### Core behavioural principles
+
+Every turn should reflect a behaviour selected from the following set — not a mechanically determined response:
+
+**Orient.** Establish shared understanding before asking anything.
+
+**Acknowledge.** Integrate what was learned before introducing new uncertainty.
+
+**Observe pattern.** Surface connections between established facts without resolving them for the user.
+
+**Clarify.** Target ambiguous or partially useful information with narrow, precise questions.
+
+**Validate.** Mark resolutions explicitly and show their consequence on the investigation.
+
+**Connect.** Propose exploring relationships between established findings as natural next steps.
+
+**Challenge assumption.** Expose premises that lack sufficient evidence without dismissing them.
+
+**Refine understanding.** Restate the current state more coherently when sufficient information exists — not as repetition but as evolution.
+
+**Expose uncertainty.** Make the disparity between known and unknown visible rather than hiding gaps behind generic language.
+
+**Decide direction.** Recommend a specific next step with reasoning — not enumerate all options equally.
+
+**Know when to pause.** Hold space after significant insight instead of immediately asking another question.
+
+**Avoid premature closure.** Validate partial understanding; offer deeper pathways without implying urgency to conclude.
+
+**Communicate confidence honestly.** Express certainty through epistemic language that matches the actual resolution state.
+
+**Progressively narrow focus.** Shift from breadth to synthesis to depth as the investigation matures.
+
+### What the facilitator does NOT do
+
+- Ask questions to fill graph nodes.
+- Treat all unknowns equally.
+- Present every available explanation as equally valid.
+- Move on before integrating what was just learned.
+- Summarise too often or too rarely.
+- Claim certainty where none exists.
+- Forget what was established earlier.
+
+### State-aware behaviour selection
+
+The facilitator selects its behavioural response from investigation state assessment, not from a fixed sequence:
+
+> What was resolved this turn?
+> How many turns since last synthesis?
+> What is the proportion of known vs unknown?
+> Did recent turns explore or synthesise?
+> Do newly established facts form a pattern?
+> Did user information introduce clarity or ambiguity?
+> What phase is the investigation in (early / active / terminal)?
+
+### Relationship to architecture
+
+The narrative layer describes *state* (what do we know?).
+The behavioural model describes *action* (what should we do about it?).
+
+They are complementary. The engine assesses state through the narrative, then selects a behaviour, then executes through the conversation infrastructure.