docs: Update README for modular meta-package architecture

Rewrite README to reflect the new v1.0 modular architecture:
- Document new pip install options (capture, ml, evals, privacy, all)
- Add unified CLI reference with all commands
- Link to individual sub-package repositories
- Remove outdated Poetry setup, old python -m commands
- Keep core value proposition and architecture diagrams
- Point to legacy/ for v0.46.0 users

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: abrichr