docs: update README and AGENTS.md with current architecture and options

- README: clarify force_launch behavior, add fixtures/PEP 723 docs, print capture info

- README: add pixi as supported project scaffolding tool

- README: remove testpaths from required config (optional, pytest collects from root by default)

- AGENTS.md: full module map, design decisions, common traps, change rules for AI agents

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: trgiangv