Redesign --sync as optional-value option (ask/do) for download and upload

[yarikoptic-gitmate]

Summary

• Redesigns --sync from a boolean flag to an optional-value option accepting ask or do, on both dandi download and dandi upload
• --sync alone (bare flag) defaults to ask — preserving full backward compatibility
• --sync do skips the interactive prompt and proceeds with deletion, enabling headless usage (CI/CD, cron jobs)
• Adds SyncMode enum in consts.py shared between download and upload
• Python API still accepts bool for backward compatibility (True normalizes to SyncMode.ASK)

Usage

dandi download --sync ...         # prompts before deleting (same as before)
dandi download --sync ask ...     # prompts (explicit)
dandi download --sync do ...      # no prompt, headless-friendly

dandi upload --sync ...           # prompts before deleting (same as before)
dandi upload --sync do ...        # no prompt

Closes #1833

Changes

• dandi/consts.py: Added SyncMode enum (ask, do)
• dandi/cli/cmd_download.py: Changed --sync from is_flag=True to optional-value with flag_value="ask" and type=Choice(["ask", "do"]); removed the separate --yes flag from the prior commit
• dandi/cli/cmd_upload.py: Same --sync redesign as download
• dandi/download.py: Changed sync parameter type to bool | SyncMode | None; when SyncMode.DO, skips abbrev_prompt
• dandi/upload.py: Changed sync parameter type to bool | SyncMode | None; when SyncMode.DO, skips click.confirm
• dandi/tests/test_download.py: Added test_download_sync_do verifying prompt is skipped
• dandi/tests/test_upload.py: Added test_upload_sync_do verifying prompt is skipped

Test plan

• All pre-commit hooks pass (black, isort, flake8, codespell)
• --sync with no value, --sync ask, --sync do, --sync=do all accepted by CLI
• --yes flag removed from download
• SyncMode enum logic verified (truthiness, normalization from bool)
• Pure unit tests pass
• test_download_sync_do and test_upload_sync_do verify prompt-free deletion
• Full integration tests require docker — skipped in this environment but follow established patterns

https://claude.ai/code/session_01297X1VGdztFArkpVFB1zEk

[codecov]

Codecov Report

❌ Patch coverage is 89.58333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.57%. Comparing base (0af950f) to head (d8f433d).

Files with missing lines	Patch %	Lines
dandi/consts.py	0.00%	5 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (0af950f) and HEAD (d8f433d). Click for more details.

HEAD has 47 uploads less than BASE

Flag	BASE (0af950f)	HEAD (d8f433d)
unittests	49	2

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1835       +/-   ##
===========================================
- Coverage   76.27%   51.57%   -24.70%     
===========================================
  Files          87       87               
  Lines       12484    12517       +33     
===========================================
- Hits         9522     6456     -3066     
- Misses       2962     6061     +3099     

Flag	Coverage Δ
unittests	51.57% <89.58%> (-24.70%)	⬇️

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