download_orbits gives error for missing path

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**Bug report**
Running ponder --download_orbits results in an error if a directory "work" doesn't already exist. We should just create it.

**Environment Information**
Fresh installation, MacOS, Python 3.12, first-go branch.

Traceback

(COC) colinchandler@Colin-MacBook-Pro-M1-UW-3 ponder % ponder --download_orbits
Fetching orbits from https://www.minorplanetcenter.net/Extended_Files/mpcorb_extended.json.gz...
Writing orbits to /Users/colinchandler/Dropbox/School/Research/ponder/work/asteroid_orbits_02-05-2026.json.gz...
Traceback (most recent call last):
File "/Users/colinchandler/opt/anaconda3/envs/COC/bin/ponder", line 6, in
sys.exit(main())
^^^^^^
File "/Users/colinchandler/GitHub/ponder/src/ponder/runner.py", line 126, in main
return cli_main()
^^^^^^^^^^
File "/Users/colinchandler/GitHub/ponder/src/ponder/__main__.py", line 26, in main
orbit_path = get_current_orbits(date_str, args.work_dir, comet=args.comet)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/colinchandler/GitHub/ponder/src/ponder/utils.py", line 249, in get_current_orbits
with open(orbits_gz_path, "wb") as f:
^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/Users/colinchandler/Dropbox/School/Research/ponder/work/asteroid_orbits_02-05-2026.json.gz'

**Before submitting**
Please check the following:

- [ ] I have described the situation in which the bug arose, including what code was executed, and any applicable data others will need to reproduce the problem.
- [ ] I have included information about my environment, including the version of this package (e.g. `ponder.__version__`)
- [ ] I have included available evidence of the unexpected behavior (including error messages, screenshots, and/or plots) as well as a description of what I expected instead.
- [ ] If I have a solution in mind, I have provided an explanation and/or pseudocode and/or task list.

Start with get_current_orbits in src/ponder/utils.py, called from src/ponder/__main__.py when handling --download_orbits. Run ponder --download_orbits with the work directory absent and confirm that the directory is created and the orbit archive is written without FileNotFoundError.

Written by the indexing model from the issue text.