Migrate tasks to native task.md packages

[bingran-you]

Summary

• migrate example_tasks/ and imported tasks/ packages to BenchFlow-native task.md layout
• replace task package solution/ and tests/ directories with native oracle/ and verifier/ directories
• update launcher/devhub/docs to read benchflow.env0.services from task.md and add a layout regression test

Validation

• python3 -m unittest tests/test_env0_control.py
• python3 devhub/app.py --render-once >/tmp/env0-devhub-render.html
• scripts/smoke_dev.sh
• cd /tmp/benchflow-task-format && uv run bench tasks check <task> --level structural for all example_tasks/* and tasks/* packages
• PULL_BASE=0 scripts/smoke_docker_examples.sh
• cd packages/environments/mock-gmail && uv run --extra dev pytest tests -q