feat(marketplace): add mattpocock-skills remote plugin

## Why
Register mattpocock/skills as an installable plugin in the holocron marketplace so users can install it with `claude plugin install mattpocock-skills@holocron`.

## Impact
Marketplace bumped from 0.1.1 → 0.2.0 (minor; new plugin entry is a backward-compatible addition).

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/plugin/marketplace.json

@@ -30,7 +30,15 @@
       "description": "Cross-cutting utility skills for everyday AI-assisted coding \u2014 triage, diagnosis, architecture review, and session navigation.",
       "name": "core",
       "source": "./plugins/core"
+    },
+    {
+      "description": "Skills for Real Engineers \u2014 planning, TDD, architecture, and debugging workflows from Matt Pocock's .claude directory.",
+      "name": "mattpocock-skills",
+      "source": {
+        "repo": "mattpocock/skills",
+        "source": "github"
+      }
     }
   ],
-  "version": "0.1.1"
+  "version": "0.2.0"
 }