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/scripts/on_user_prompt.sh

@@ -25,14 +25,14 @@ if [ ${#PROMPT} -lt 20 ]; then
   exit 0
 fi
 
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+# shellcheck source=_identity.sh
+. "$SCRIPT_DIR/_identity.sh"
+
 # No API key means the agent can't search anyway
 if [ -z "${MEM0_API_KEY:-}" ]; then
   exit 0
 fi
-
-SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-# shellcheck source=_identity.sh
-. "$SCRIPT_DIR/_identity.sh"
 USER_ID="$MEM0_RESOLVED_USER_ID"
 
 cat <<EOF