exp(06): focused investigation — visual hierarchy without layout changes

Emphasise the active investigation card through stronger elevation,
clearer borders, and improved spacing. Quiet supporting panels by
reducing border opacity, softening heading weight, and lowering
text contrast — making them available without competing for attention.

Facilitator card receives a warm surface tint to read as a briefing
card rather than a generic panel.

Documentation: close experiment 05 with findings, add experiment 06
to the design evolution log, add Attention Hierarchy to UX guidelines,
defer dark mode to a future Investigation Mode experiment.

Presentation changes only — no reasoning, prompts, graph, API, or
backend modifications.

.claude/ux-guidelines.md

@@ -348,3 +348,24 @@ Returning users should control repeated guidance.
 The workspace should remain the primary visual focus.
 
 Information should naturally flow from left to right.
+
+## Attention Hierarchy
+
+The current task always owns the user's attention.
+
+Supporting information should remain available without competing.
+
+Visual emphasis should come primarily from hierarchy rather than colour.
+
+Reduce distraction before adding decoration.
+
+Calm interfaces improve reasoning.
+
+Hierarchy flows from strongest to quietest:
+
+1. The current investigation question (strongest visual element)
+2. The response area (interactive, clear action)
+3. Supporting context (visible but restrained)
+4. Reference material (available, low priority)
+
+The workspace should feel like an active desk — the work in progress is prominent, supporting tools are within reach but not shouting for attention.