Document Python 3.12 CLI install requirement

[bingran-you]

Summary

• update public CLI install commands to pin the uv tool interpreter with --python 3.12
• explain why Python 3.10/3.11 can resolve the old no-entrypoint benchflow==0.1.13 wheel
• add a docs drift guard requiring documented uv tool install ... benchflow commands to include --python 3.12
• fix the Daytona DinD heartbeat wrapper so fast commands terminate promptly instead of waiting on a background sleep

Verification

• fresh Docker repro: Python 3.11 + uv tool install --upgrade benchflow selects benchflow==0.1.13 and fails with No executables are provided by package benchflow
• fresh Docker control: Python 3.12 + same command installs benchflow==0.6.4 with bench and benchflow
• fresh Docker fix: Python 3.11 + uv tool install --python 3.12 --upgrade benchflow installs benchflow==0.6.4 with bench
• uv run python -m pytest tests/test_cli_docs_drift.py -q
• uv run python -m pytest tests/test_daytona_dind_compose_up.py -q
• uv run ruff check src/benchflow/sandbox/daytona_dind.py tests/test_cli_docs_drift.py tests/test_daytona_dind_compose_up.py
• uv run ruff format --check src/benchflow/sandbox/daytona_dind.py tests/test_cli_docs_drift.py tests/test_daytona_dind_compose_up.py
• git diff --check

Fixes #898