feat: add model field validation to marketplace script

Add validation for:
- defaultModel field in plugin.json (must be opus|sonnet|haiku)
- model field in agent frontmatter (must be opus|sonnet|haiku)

The validation passes when fields are absent (optional) but errors
if present with invalid values.

Fixes #306

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

scripts/validate-marketplace.sh

@@ -143,6 +143,16 @@ 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 $plugin_name/"
@@ -151,6 +161,40 @@ for plugin_dir in "$PLUGINS_DIR"/*/; do
     echo "✓ $plugin_name valid"
 done
 
+# 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+) ==="