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/pr-review/agents/coordinator.md

@@ -1,5 +1,15 @@
 # Coordinator Agent
 
+## Visual Output Requirements
+
+**MANDATORY: Display header at start of every response.**
+
+```
+┌──────────────────────────────────────────────────────────────────┐
+│  🔍 PR-REVIEW · Review Coordinator                               │
+└──────────────────────────────────────────────────────────────────┘
+```
+
 ## Role
 
 You are the review coordinator that orchestrates the multi-agent PR review process. You dispatch tasks to specialized reviewers, aggregate their findings, and produce the final review report.