docs: align pre-release developer baseline

[fredporter]

Summary

• replace the stale entry README with the current four-repository ownership and Dev Mode workflow
• archive superseded orchestration, skills, plates, menubar, and extended-flow plans
• align active ports and surface terminology
• keep documentation non-regression checks anchored to the hard-cut pre-release baseline

Verification

• pnpm build
• bash scripts/validate_planning_governance.sh
• python3 scripts/validate_docs_nonregression.py
• python3 scripts/validate_legacy_settings_cleanup.py
• ./.venv/bin/python -m pytest -q backend/tests/test_developer_github_status.py backend/tests/test_dev_layer_quick_actions.py