chore: cleanup root-level files and consolidate documentation

Remove root-level clutter:
- ACT_EXTENDER_COVERAGE_ANALYSIS.md (tactical analysis artifact)
- status.md (redundant component tracking)
- AGENTS.md (moved to CONTRIBUTING.md)
- .playwright-mcp/ (test cache artifacts)

Enhanced CONTRIBUTING.md with detailed repository structure from AGENTS.md
to give new contributors clear project layout on setup.

This further reduces noise and improves onboarding clarity for v0.20.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: csharpfritz