feat: graduate wizard into the engineering bucket

Move `wizard` out of in-progress into `engineering/` and wire it up as a
promoted skill: plugin.json entry, top-level + Engineering READMEs under
User-invoked, a docs page at docs/engineering/wizard.md, and a Standalone
route in ask-matt for the steps only a human can take.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -36,6 +36,7 @@
     "./skills/engineering/codebase-design",
     "./skills/engineering/code-review",
     "./skills/engineering/resolving-merge-conflicts",
+    "./skills/engineering/wizard",
     "./skills/productivity/grill-me",
     "./skills/productivity/grilling",
     "./skills/productivity/handoff",