Plugin: add memory_recall system guidance, fix autoRecall query

[Teingi]

• Inject prependSystemContext so the agent is told to use memory_recall or
  when answering about past events or preferences.
• Use last user message as recall query when prompt is too short.
• Always return guidance when autoRecall is on (even on no results or error).
• Docs: add troubleshooting for agent not searching memory and for
  memory/YYYY-MM-DD.md ENOENT (disable session-memory); clarify that
  npm i does not register the plugin and that openclaw plugins install
  downloads from the npm registry.