release: v3.28.1 - Visual Diagrams Series (40 Mermaid diagrams)

guide/diagrams/: new directory with 40 interactive Mermaid diagrams
- 10 thematic files: foundations, context/sessions, configuration,
  architecture, MCP ecosystem, dev workflows, multi-agent patterns,
  security/production, cost/optimization, adoption/learning
- Each diagram: Mermaid (GitHub-native) + ASCII fallback + source link
- Bold Guy palette (6-color system) consistent across all diagrams
- README with index, visual palette legend, navigation by use case

Also includes (backlog from v3.28.0→v3.28.1):
- guide/ultimate-guide.md: Managing Large MCP Server Sets, AI Code
  Disclosure Policy, claude-mem Gemini alternative, observability
- guide/workflows/plan-driven.md: Boris Tane custom markdown plans (+172L)
- guide/security-hardening.md: Part 4 PR security review workflow
- examples/agents/security-patcher.md: new security agent
- examples/hooks/bash/security-gate.sh: PreToolUse security hook
- guide/observability.md: activity monitoring, external tools, proxying
- docs/resource-evaluations/: 4 new evaluations (Boris Cherny, Moigneu,
  Boris Tane, Aristote AI instructions)
- README.md: Visual Diagrams section in "What Makes This Guide Unique"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

guide/visual-reference.md

@@ -9,6 +9,8 @@ tags: [reference, architecture, cheatsheet]
 All diagrams in one place. Quick visual overview of Claude Code's key concepts.
 For detailed docs → [Ultimate Guide](./ultimate-guide.md) | [Cheatsheet](./cheatsheet.md)
 
+> **Interactive Mermaid diagrams available**: For 40 interactive diagrams covering model selection, agent lifecycle, memory hierarchy, multi-agent patterns, security threats, and more — see **[guide/diagrams/](./diagrams/)**. This file contains ASCII versions of key concepts.
+
 > **20 diagrams**: 8 new (this file) + 12 from existing guides, all consolidated here.
 
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