fix(mem0-plugin): resolve API key from userConfig, fix #4876

Root cause: .mcp.json required ${MEM0_API_KEY} shell env var which
OAuth flow never populates — MCP server never started, only auth
stubs were exposed.

Fix: switch .mcp.json to ${user_config.MEM0_API_KEY} so Claude Code
reads the key from plugin userConfig (system keychain). Add
resolve_api_key() fallback chain (MEM0_API_KEY -> CLAUDE_PLUGIN_OPTION_MEM0_API_KEY)
to all hook scripts. Update onboard skill and setup_coding_categories.py
to handle keychain-only users. Add 3 tests for key resolution.

mem0-plugin/.mcp.json

@@ -4,7 +4,7 @@
       "type": "http",
       "url": "https://mcp.mem0.ai/mcp/",
       "headers": {
-        "Authorization": "Token ${MEM0_API_KEY}"
+        "Authorization": "Token ${user_config.MEM0_API_KEY}"
       }
     }
   }