chore(release): plugin.json version 0.20.0

New skill (document-to-markdown) is a minor bump; the gate asserts
manifest/tag parity, so this lands with the tag.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "skills",
   "description": "Mathias's engineering skills library, plus the hooks and MCP servers that make it enforceable: TDD, clean code, debugging, specs, discovery, CI, and secret hygiene.",
-  "version": "0.19.6",
+  "version": "0.20.0",
   "author": {
     "name": "Mathias",
     "url": "https://git.d-ma.be/mathias"