CLI: support worker-based execution

[atravitz]

LLM / AI generated code disclosure

LLMs or other AI-powered tools (beyond simple IDE use cases) were used in this contribution: no

Checklist

• All new code is appropriately documented (user-facing code must have complete docstrings).
• Added a news entry, or the changes are not user-facing.
• Ran pre-commit: you can run pre-commit locally or comment on this PR with pre-commit.ci autofix.
• Filled in the AI generated code disclosure.

Manual Tests: these are slow so don't need to be run every commit, only before merging and when relevant changes are made (generally at reviewer-discretion).

• GPU integration tests
• example notebook testing
• packaging tests: run this for any large feature PRs or PRs that add test data.

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov]

Codecov Report

❌ Patch coverage is 97.25275% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.53%. Comparing base (755cfd1) to head (fd2f31f).
⚠️ Report is 22 commits behind head on epic/execution_improvements.

Files with missing lines	Patch %	Lines
src/openfecli/commands/run_task.py	80.00%	3 Missing ⚠️
src/openfecli/commands/to_legacy_json.py	95.83%	1 Missing ⚠️
src/openfecli/plan_alchemical_networks_utils.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                       @@
##           epic/execution_improvements    #2174      +/-   ##
===============================================================
+ Coverage                        90.24%   90.53%   +0.29%     
===============================================================
  Files                              209      222      +13     
  Lines                            20475    21540    +1065     
===============================================================
+ Hits                             18477    19502    +1025     
- Misses                            1998     2038      +40     

Flag	Coverage Δ
fast-tests	90.53% <97.25%> (+0.29%)	⬆️

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