Support no-LLM mode

[wayyoungboy]

Summary

• Add a noop LLM provider for explicitly disabled LLM mode.
• Preserve basic memory add/search/update/delete while skipping fact extraction, action planning, graph extraction/search, profile extraction, query rewrite, compression, skill distillation/merge, and LLM-based importance scoring.
• Let the Claude Code plugin init fall back to no-LLM mode when LLM credentials are missing or incomplete, and report disabled LLM status in CLI/health checks.

Validation

• python -m pytest tests/unit/test_noop_llm.py tests/integration/test_noop_llm_mode.py -q
• python -m pytest tests/unit/test_noop_llm.py tests/integration/test_noop_llm_mode.py tests/unit/test_config_loader.py tests/unit/test_memory.py tests/unit/intelligence/test_memory_optimizer.py tests/unit/intelligence/test_intelligent_memory_manager.py tests/unit/server/test_user_service.py -q
• python -m compileall -q src/powermem/integrations/llm src/powermem/core src/powermem/user_memory src/powermem/intelligence src/powermem/agent/components src/server apps/claude-code-plugin/scripts
• sh -n apps/claude-code-plugin/scripts/init.sh && git diff --check