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/examples/read-settings-hook.sh

@@ -0,0 +1,65 @@
+#!/bin/bash
+# Example hook that reads plugin settings from .claude/my-plugin.local.md
+# Demonstrates the complete pattern for settings-driven hook behavior
+
+set -euo pipefail
+
+# Define settings file path
+SETTINGS_FILE=".claude/my-plugin.local.md"
+
+# Quick exit if settings file doesn't exist
+if [[ ! -f "$SETTINGS_FILE" ]]; then
+  # Plugin not configured - use defaults or skip
+  exit 0
+fi
+
+# Parse YAML frontmatter (everything between --- markers)
+FRONTMATTER=$(sed -n '/^---$/,/^---$/{ /^---$/d; p; }' "$SETTINGS_FILE")
+
+# Extract configuration fields
+ENABLED=$(echo "$FRONTMATTER" | grep '^enabled:' | sed 's/enabled: *//' | sed 's/^"\(.*\)"$/\1/')
+STRICT_MODE=$(echo "$FRONTMATTER" | grep '^strict_mode:' | sed 's/strict_mode: *//' | sed 's/^"\(.*\)"$/\1/')
+MAX_SIZE=$(echo "$FRONTMATTER" | grep '^max_file_size:' | sed 's/max_file_size: *//')
+
+# Quick exit if disabled
+if [[ "$ENABLED" != "true" ]]; then
+  exit 0
+fi
+
+# Read hook input
+input=$(cat)
+file_path=$(echo "$input" | jq -r '.tool_input.file_path // empty')
+
+# Apply configured validation
+if [[ "$STRICT_MODE" == "true" ]]; then
+  # Strict mode: apply all checks
+  if [[ "$file_path" == *".."* ]]; then
+    echo '{"hookSpecificOutput": {"permissionDecision": "deny"}, "systemMessage": "Path traversal blocked (strict mode)"}' >&2
+    exit 2
+  fi
+
+  if [[ "$file_path" == *".env"* ]] || [[ "$file_path" == *"secret"* ]]; then
+    echo '{"hookSpecificOutput": {"permissionDecision": "deny"}, "systemMessage": "Sensitive file blocked (strict mode)"}' >&2
+    exit 2
+  fi
+else
+  # Standard mode: basic checks only
+  if [[ "$file_path" == "/etc/"* ]] || [[ "$file_path" == "/sys/"* ]]; then
+    echo '{"hookSpecificOutput": {"permissionDecision": "deny"}, "systemMessage": "System path blocked"}' >&2
+    exit 2
+  fi
+fi
+
+# Check file size if configured
+if [[ -n "$MAX_SIZE" ]] && [[ "$MAX_SIZE" =~ ^[0-9]+$ ]]; then
+  content=$(echo "$input" | jq -r '.tool_input.content // empty')
+  content_size=${#content}
+
+  if [[ $content_size -gt $MAX_SIZE ]]; then
+    echo '{"hookSpecificOutput": {"permissionDecision": "deny"}, "systemMessage": "File exceeds configured max size: '"$MAX_SIZE"' bytes"}' >&2
+    exit 2
+  fi
+fi
+
+# All checks passed
+exit 0