refactor(plugin): self-maintaining directory form for plugin skills

Replace the hand-listed 22 skill directories in
.claude-plugin/plugin.json with the two promoted buckets
(./skills/engineering, ./skills/productivity). Claude Code
discovers skills one level down, so newly promoted skills are
picked up automatically and the manifest can't drift out of sync
with the tree. Resolves the same enumeration gap that once left
resolving-merge-conflicts off the list.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.changeset/plugin-skills-directory-form.md

@@ -0,0 +1,5 @@
+---
+"mattpocock-skills": patch
+---
+
+Switch the plugin manifest's `skills` field to self-maintaining directory form. Instead of hand-listing all 22 skill directories, `.claude-plugin/plugin.json` now points at the two promoted buckets (`./skills/engineering`, `./skills/productivity`) and lets Claude Code discover the skills one level down. New promoted skills are picked up automatically, and the manifest can no longer drift out of sync with the tree.

.claude-plugin/plugin.json

@@ -19,27 +19,7 @@
     "productivity"
   ],
   "skills": [
-    "./skills/engineering/ask-matt",
-    "./skills/engineering/diagnosing-bugs",
-    "./skills/engineering/grill-with-docs",
-    "./skills/engineering/triage",
-    "./skills/engineering/improve-codebase-architecture",
-    "./skills/engineering/setup-matt-pocock-skills",
-    "./skills/engineering/tdd",
-    "./skills/engineering/to-spec",
-    "./skills/engineering/to-tickets",
-    "./skills/engineering/wayfinder",
-    "./skills/engineering/implement",
-    "./skills/engineering/prototype",
-    "./skills/engineering/research",
-    "./skills/engineering/domain-modeling",
-    "./skills/engineering/codebase-design",
-    "./skills/engineering/code-review",
-    "./skills/engineering/resolving-merge-conflicts",
-    "./skills/productivity/grill-me",
-    "./skills/productivity/grilling",
-    "./skills/productivity/handoff",
-    "./skills/productivity/teach",
-    "./skills/productivity/writing-great-skills"
+    "./skills/engineering",
+    "./skills/productivity"
   ]
 }