chore: release v5.9.0

- Plugin installation scripts for consumer projects
- MCP server mapping via mcp_servers field in plugin.json
- CLAUDE.md section markers for install/uninstall
- Agent model selection (25 agents with model frontmatter)
- Agent frontmatter standardization

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

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
+---
+
+## [5.9.0] - 2026-02-03
+
 ### Added
 
 #### Plugin Installation Scripts