Wire mattpocock skills into engineering docs, fix stale misc README

Adds the 8 already-copied engineering skills (ask-matt, codebase-design,
domain-modeling, implement, prototype, resolving-merge-conflicts,
setup-matt-pocock-skills, triage) plus frontend-design to
engineering/README.md and plugin.json. Also rewrites misc/README.md,
which listed 3 skills with no matching folder and was missing 3 real ones.

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

.claude-plugin/plugin.json

@@ -8,6 +8,14 @@
     "./skills/engineering/to-issues",
     "./skills/engineering/to-prd",
     "./skills/engineering/frontend-design",
+    "./skills/engineering/ask-matt",
+    "./skills/engineering/codebase-design",
+    "./skills/engineering/domain-modeling",
+    "./skills/engineering/implement",
+    "./skills/engineering/prototype",
+    "./skills/engineering/resolving-merge-conflicts",
+    "./skills/engineering/setup-matt-pocock-skills",
+    "./skills/engineering/triage",
     "./skills/productivity/grill-me",
     "./skills/productivity/grilling",
     "./skills/productivity/handoff"