docs: Complete README overhaul with parallel LLM examples and memory docs

Major updates:
- Rewrote README with simple explanations + parallel ensemble examples
- Added multi-provider parallel execution examples (Groq + OpenAI + DeepSeek)
- Added memory capability documentation (semantic cache, context, cross-session)
- Added LangChain, LlamaIndex, CrewAI integration examples
- Created llms.txt (LLM indexable summary)
- Created docs/llms-full.txt (full technical reference)
- Added 100+ keywords to package.json
- Updated package to v2.15.4

New README structure:
- TL;DR quick start
- Parallel ensemble examples (not just OpenAI)
- Cost comparison tables
- Memory system documentation
- Multi-agent examples (CrewAI)
- Provider coverage table
- CLI commands

Author: Subho Mukherjee