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.

docs/design-evolution-log.md

@@ -735,20 +735,74 @@ It is intentionally recorded before implementation so future experiments remain
 
 ---
 
+### Experiment 15 — Facilitator Behaviour Specification
+
+#### Hypothesis
+
+An expert consultant does not have a script. They have behaviours — recurring patterns of action deployed based on what they observe in the client's situation. The Confidence Engine should exhibit similar behavioural patterns rather than following a mechanical question-fill-graph cycle.
+
+The current engine behaviour is:
+
+> Engine asks → User answers → Graph updates → Engine asks again
+
+An expert facilitator behaviour is:
+
+> Engine assesses state → selects appropriate behaviour → acts (question, acknowledge, synthesise, challenge, pause)
+
+#### Questions
+
+- How does an expert consultant behave during an investigation?
+- Which behaviours recur across investigations?
+- What triggers each behaviour?
+- When does the facilitator ask a question versus summarise versus expose uncertainty versus hold space?
+- What distinguishes guided thinking from mechanical Q&A?
+
+#### Status
+
+Investigation — behavioural model documented, not yet implemented.
+
+#### Evaluation
+
+This experiment is primarily architectural and behavioural. No code changes are required at this stage. The deliverable is a behavioural specification that future implementation experiments will reference.
+
+#### Result
+
+Confirmed as the correct next direction.
+
+#### What did we learn?
+
+- Every visual and architectural question has been answered by Experiment 14. Further visual iteration yields diminishing returns.
+- The remaining gap is not visual — it is behavioural.
+- The engine's behaviour pattern is fundamentally different from an expert consultant: mechanical Q&A versus adaptive, state-aware facilitation.
+- The graph captures *state* but not *behaviour*. It records what is known and what remains uncertain, but not how understanding developed across turns.
+- Conversation rhythm matters more than panel labels for creating the experience of genuine facilitated thinking.
+- 14 distinct facilitator behaviours were identified: 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.
+- Each behaviour has specific triggers and conditions mapped to investigation state.
+- The engine's turn cycle should shift from "assess unknown → ask question" to "assess state → select behaviour → act".
+
+#### Decision
+
+Commit the behavioural specification. Do not implement yet. Future experiments will integrate behavioural assessment into the reasoning cycle. This document defines what the facilitator does; future work determines how the system implements it.
+
+---
+
 ## Emerging Direction — Graph as Source of Truth
 
 The first UX experiments focused on workspace structure.
 
-The next series will focus on investigation rhythm.
+The next series will focus on investigation rhythm and behaviour.
 
 Future experiments should explore:
 
-- how conversations unfold
-- how understanding evolves
-- how transitions feel
-- how confidence is gradually built
+- how conversations unfold (behavioural, not visual)
+- how understanding evolves across turns
+- how the facilitator selects its behavioural response
+- how confidence is gradually built through action, not description
+- what state assessment enables better question selection
 
-The objective is no longer to arrange cards.
+The objective is no longer to arrange cards or translate panels.
+
+The objective is to make each turn of the investigation feel like a natural step in a guided thinking process.
 
 The objective is to make the investigation feel like a natural facilitated conversation.