feat: Add plugin-dev toolkit for comprehensive plugin development

Adds the plugin-dev plugin to public marketplace. A comprehensive toolkit for
developing Claude Code plugins with 7 expert skills, 3 AI-assisted agents, and
extensive documentation covering the complete plugin development lifecycle.

Key features:
- 7 skills: hook-development, mcp-integration, plugin-structure, plugin-settings,
  command-development, agent-development, skill-development
- 3 agents: agent-creator (AI-assisted generation), plugin-validator (structure
  validation), skill-reviewer (quality review)
- 1 command: /plugin-dev:create-plugin (guided 8-phase workflow)
- 10 utility scripts for validation and testing
- 21 reference docs with deep-dive guidance (~11k words)
- 9 working examples demonstrating best practices

Changes for public release:
- Replaced all references to internal repositories with "Claude Code"
- Updated MCP examples: internal.company.com → api.example.com
- Updated token variables: ${INTERNAL_TOKEN} → ${API_TOKEN}
- Reframed agent-creation-system-prompt as "proven in production"
- Preserved all ${CLAUDE_PLUGIN_ROOT} references (186 total)
- Preserved valuable test blocks in core modules

Validation:
- All 3 agents validated successfully with validate-agent.sh
- All JSON files validated with jq
- Zero internal references remaining
- 59 files migrated, 21,971 lines added

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

plugins/plugin-dev/skills/plugin-settings/scripts/parse-frontmatter.sh

@@ -0,0 +1,59 @@
+#!/bin/bash
+# Frontmatter Parser Utility
+# Extracts YAML frontmatter from .local.md files
+
+set -euo pipefail
+
+# Usage
+show_usage() {
+  echo "Usage: $0 <settings-file.md> [field-name]"
+  echo ""
+  echo "Examples:"
+  echo "  # Show all frontmatter"
+  echo "  $0 .claude/my-plugin.local.md"
+  echo ""
+  echo "  # Extract specific field"
+  echo "  $0 .claude/my-plugin.local.md enabled"
+  echo ""
+  echo "  # Extract and use in script"
+  echo "  ENABLED=\$($0 .claude/my-plugin.local.md enabled)"
+  exit 0
+}
+
+if [ $# -eq 0 ] || [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
+  show_usage
+fi
+
+FILE="$1"
+FIELD="${2:-}"
+
+# Validate file
+if [ ! -f "$FILE" ]; then
+  echo "Error: File not found: $FILE" >&2
+  exit 1
+fi
+
+# Extract frontmatter
+FRONTMATTER=$(sed -n '/^---$/,/^---$/{ /^---$/d; p; }' "$FILE")
+
+if [ -z "$FRONTMATTER" ]; then
+  echo "Error: No frontmatter found in $FILE" >&2
+  exit 1
+fi
+
+# If no field specified, output all frontmatter
+if [ -z "$FIELD" ]; then
+  echo "$FRONTMATTER"
+  exit 0
+fi
+
+# Extract specific field
+VALUE=$(echo "$FRONTMATTER" | grep "^${FIELD}:" | sed "s/${FIELD}: *//" | sed 's/^"\(.*\)"$/\1/' | sed "s/^'\\(.*\\)'$/\\1/")
+
+if [ -z "$VALUE" ]; then
+  echo "Error: Field '$FIELD' not found in frontmatter" >&2
+  exit 1
+fi
+
+echo "$VALUE"
+exit 0