Incorporate ai-config skills via plugin marketplace

Enable the d-morrison/ai-config plugin marketplace in this repo so the
shared workflow skills (PR/review automation, grading, reprexes, R-package
helpers, etc.) are available in Claude Code on the web sessions opened on
psw, and to the @claude CI bot (claude-code-action reads this project
settings file from the checked-out repo).

This is the consumer-repo mechanism documented in ai-config's README:
skills stay single-sourced in d-morrison/ai-config and are pulled in here
by reference, with no duplication.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018v7h9ZVVQRBjdyRFGXCrzG

Author: claude

.claude/settings.json

@@ -0,0 +1,10 @@
+{
+  "extraKnownMarketplaces": {
+    "d-morrison": {
+      "source": { "source": "github", "repo": "d-morrison/ai-config" }
+    }
+  },
+  "enabledPlugins": {
+    "ai-config@d-morrison": true
+  }
+}