Dogfood the Action + bump to v20.0.0 (Agentic Tooling) (#42)

- .github/workflows/pr-description.yml: uses our own Action (uses: ./action)
  to auto-write this repo's PR descriptions when a PR opens empty; skips
  quietly without ANTHROPIC_API_KEY and on forks. A living demo.
- Version -> 20.0.0 (Agentic Tooling): bundles the GitHub Action, generate
  command, and evals/leaderboard for npm. README badge + What's New (v19
  collapsed), CHANGELOG [Unreleased] -> [20.0.0], SECURITY table.


Claude-Session: https://claude.ai/code/session_016JWn5jRD5tcEFKrubjQ6Px

Co-authored-by: Claude <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "pm-claude-skills",
-  "version": "19.0.0",
+  "version": "20.0.0",
   "type": "module",
   "description": "167 professional Agent Skills (SKILL.md) + subagents + slash commands for Claude, ChatGPT, Gemini, Cursor, Codex & Hermes. Install into any AI coding tool with: npx pm-claude-skills add --agent <tool>.",
   "keywords": [