chore(marketplace): add ourdigital-skills local marketplace + fix README install docs

Add .claude-plugin/marketplace.json: the ourdigital-skills local directory marketplace — 11 plugins. 8 themed skill plugins (core/seo/notion/jamie/notebooklm/gtm/dintel/utils) bundle 66 skills via custom-skills/NN-*/ roots; 3 standalone plugins (mac-optimizer, multi-agent-guide, dintel-bootstrap) expose their own commands/ + skills/. Whole manifest passes 'claude plugin validate'.

Fix mac-optimizer + multi-agent-guide plugin.json: drop the invalid object-form 'commands' field ({name,description} objects; the schema expects command path strings) so commands auto-discover from each plugin's commands/ directory.

Fix README install instructions: clarify Quick Install scope (commands + venv + creds, not skills); correct the Claude Code skill symlink target to the skill root (was desktop/, which would not load); fix the Claude Desktop import step.

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

custom-skills/91-multi-agent-guide/.claude-plugin/plugin.json

@@ -18,15 +18,5 @@
     "gemini",
     "codex"
   ],
-  "license": "MIT",
-  "commands": [
-    {
-      "name": "multi-agent-setup",
-      "description": "Quick setup for multi-agent collaboration"
-    },
-    {
-      "name": "setup-agents",
-      "description": "Full interactive multi-agent setup"
-    }
-  ]
+  "license": "MIT"
 }