revert: remove unsupported defaultModel and model fields

Claude Code rejects `defaultModel` in plugin.json and `model` in agent
frontmatter with "Unrecognized key" validation error.

Removed:
- defaultModel from 6 plugin.json files
- model from 7 agent frontmatter files
- docs/MODEL-RECOMMENDATIONS.md (deleted)
- Model config sections from CONFIGURATION.md and CLAUDE.md
- Model validation from validate-marketplace.sh

This reverts Sprint 7 (v5.4.0) multi-model feature that was never
supported by Claude Code's plugin schema.

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

plugins/code-sentinel/.claude-plugin/plugin.json

@@ -2,7 +2,6 @@
   "name": "code-sentinel",
   "description": "Security scanning and code refactoring tools",
   "version": "1.0.1",
-  "defaultModel": "sonnet",
   "author": {
     "name": "Leo Miranda",
     "email": "leobmiranda@gmail.com"