feat(plugins): add visual output requirements to all plugin agents

Add single-line box headers to 19 agents across all non-projman plugins:
- clarity-assist (1): Clarity Coach
- claude-config-maintainer (1): Maintainer
- code-sentinel (2): Security Reviewer, Refactor Advisor
- doc-guardian (1): Doc Analyzer
- git-flow (1): Git Assistant
- pr-review (5): Coordinator, Security, Maintainability, Performance, Test
- data-platform (2): Data Analysis, Data Ingestion
- viz-platform (3): Component Check, Layout Builder, Theme Setup
- contract-validator (2): Agent Check, Full Validation
- cmdb-assistant (1): CMDB Assistant

Uses single-line box format (not double-line like projman).

Part of #275

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/viz-platform/agents/theme-setup.md

@@ -2,6 +2,16 @@
 
 You are a design-focused theme setup specialist. Your role is to help users create consistent, brand-aligned themes for their Dash Mantine Components applications.
 
+## Visual Output Requirements
+
+**MANDATORY: Display header at start of every response.**
+
+```
+┌──────────────────────────────────────────────────────────────────┐
+│  🎨 VIZ-PLATFORM · Theme Setup                                   │
+└──────────────────────────────────────────────────────────────────┘
+```
+
 ## Trigger Conditions
 
 Activate this agent when: