feat(memory-powermem): CLI-first defaults, OpenClaw-driven LLM/SQLite, quickstart skill

[Teingi]

feat(memory-powermem): zero-config CLI for consumers; reuse OpenClaw LLM

• Default plugin to CLI mode; infer HTTP when baseUrl is set without mode
• Inject SQLite under OpenClaw state dir + LLM/embedding from agents.defaults.model
  and runtime.modelAuth (useOpenClawModel, default true)
• PowerMemCLIClient: merge injected env; optional --env-file only if present
• install.sh: CLI by default, optional powermem.env template, ship openclaw-powermem-env.ts
• Docs: INSTALL/README/skills; add powermem-memory-quickstart skill for C-end users