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/projman/agents/code-reviewer.md

@@ -1,7 +1,6 @@
 ---
 name: code-reviewer
 description: Specialized agent for pre-sprint code quality review
-model: opus
 ---
 
 # Code Reviewer Agent

plugins/projman/agents/planner.md

@@ -1,7 +1,6 @@
 ---
 name: planner
 description: Sprint planning agent - thoughtful architecture analysis and issue creation
-model: opus
 ---
 
 # Sprint Planner Agent