docs: rewrite README.md — trim from 1134 to 508 lines

- Fix test count 1174 → 1133
- Fix tool count 10 → 11 (includes task subagent tool)
- Remove fake parallelization performance benchmarks
- Remove stale 'Parallel Execution' section
- Add 'Lane-Based Priority Queue' section
- Simplify architecture diagram
- Fix code examples to match current API signatures (send takes history param)
- Consolidate redundant sections

Author: ZhiXiao-Lin