fix(fixer): add design-boundary constraint to fixer prompt

The fixer's system prompt listed constraints (no research, no
delegation) but never said it shouldn't do design work. When the
orchestrator model (or a direct @fixer call) routed UI tasks to fixer,
it would implement them with whatever taste its model had — usually
generic SaaS slop.

One line in the Constraints section closes the gap: no layout, styling,
visual hierarchy, responsive behavior, animation, or component feel.
These belong to @designer.

Fixes #444

src/agents/fixer.ts

@@ -18,11 +18,12 @@ ${WRITABLE_FILE_OPERATIONS_RULES}
 
 **Constraints**:
 - NO external research (no websearch, context7, gh_grep)
-- NO delegation or spawning subagents
+- NO spawning subagents; telling the caller which specialist to use is fine
 - No multi-step research/planning; minimal execution sequence ok
 - If context is insufficient: use grep/glob/read directly - do not delegate
 - Only ask for missing inputs you truly cannot retrieve yourself
 - Do not act as the primary reviewer; implement requested changes and surface obvious issues briefly
+- No design work — layout, styling, visual hierarchy, responsive behavior, animation, component feel. Refuse and tell the caller to use @designer.
 
 **Output Format**:
 <summary>

src/agents/orchestrator.ts

@@ -162,7 +162,8 @@ Review available agents and lane rules. Before beginning non-trivial work, ident
 
 **Routing threshold:**
 - Handle directly only for one isolated, clear, low-risk action where delegation would cost more than execution.
-- For multi-step implementation, broad discovery, external research, visual work, or complex debugging, delegate to the suitable specialist.
+- Never handle UI/design work directly — layout, styling, visual hierarchy, responsive behavior, animation, and component feel always route to @designer.
+- For multi-step implementation, broad discovery, external research, or complex debugging, delegate to the suitable specialist.
 - If two or more parts can proceed independently, dispatch them in parallel before starting dependent work.
 - Do not delegate merely because an agent exists. Do not keep substantive work entirely in the orchestrator merely because each individual step seems easy.