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>

scripts/validate-marketplace.sh

@@ -143,16 +143,6 @@ for plugin_dir in "$PLUGINS_DIR"/*/; do
         echo "WARNING: Missing 'keywords' array in $plugin_name/plugin.json"
     fi
 
-    # v5.4.0: Validate defaultModel field if present
-    default_model=$(jq -r '.defaultModel // empty' "$plugin_json")
-    if [[ -n "$default_model" ]]; then
-        if [[ ! "$default_model" =~ ^(opus|sonnet|haiku)$ ]]; then
-            echo "ERROR: Invalid defaultModel '$default_model' in $plugin_name/plugin.json (must be opus|sonnet|haiku)"
-            exit 1
-        fi
-        echo "  ✓ defaultModel: $default_model"
-    fi
-
     # Check README exists
     if [[ ! -f "$plugin_dir/README.md" ]]; then
         echo "WARNING: Missing README.md in $plugin_name/"
@@ -258,40 +248,6 @@ done
 
 echo "✓ All file references validated"
 
-# v5.4.0: Validate agent model fields
-echo ""
-echo "=== Validating Agent Model Fields (v5.4.0+) ==="
-
-validate_agent_model() {
-    local file="$1"
-    local agent_name=$(basename "$file" .md)
-
-    # Extract model from frontmatter (between --- markers)
-    local model=$(sed -n '/^---$/,/^---$/p' "$file" | grep '^model:' | awk '{print $2}')
-
-    if [[ -n "$model" ]]; then
-        if [[ ! "$model" =~ ^(opus|sonnet|haiku)$ ]]; then
-            echo "ERROR: Invalid model '$model' in $file (must be opus|sonnet|haiku)"
-            exit 1
-        fi
-        echo "  ✓ $agent_name: model=$model"
-    fi
-}
-
-for plugin_dir in "$PLUGINS_DIR"/*/; do
-    plugin_name=$(basename "$plugin_dir")
-    agents_dir="$plugin_dir/agents"
-
-    if [[ -d "$agents_dir" ]]; then
-        echo "--- Checking agents in $plugin_name ---"
-        for agent_file in "$agents_dir"/*.md; do
-            if [[ -f "$agent_file" ]]; then
-                validate_agent_model "$agent_file"
-            fi
-        done
-    fi
-done
-
 # v3.0.0: Validate MCP server symlinks
 echo ""
 echo "=== Validating MCP Server Symlinks (v3.0.0+) ==="