feat: replace support of "datetime_filesafe" format field in output file prefix format string with "datetime"

[candleindark]

This PR closes #304

[Copilot]

Pull request overview

This PR replaces the datetime_filesafe format field with datetime in the output file prefix format string throughout the codebase. The change maintains backwards compatibility by still accepting datetime_filesafe in the LogPaths.create() method while promoting datetime as the new standard. The actual datetime format remains unchanged (still file-safe: %Y.%m.%dT%H.%M.%S).

Changes:

• Updated default DUCT_OUTPUT_PREFIX to use {datetime} instead of {datetime_filesafe}
• Modified LogPaths.create() to support both {datetime} and {datetime_filesafe} format fields for backwards compatibility
• Updated documentation in CLI help text and README to reflect the new {datetime} field name

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/con_duct/duct_main.py	Updated default prefix constant and modified LogPaths.create() to accept both datetime and deprecated datetime_filesafe fields
src/con_duct/cli.py	Updated CLI help text to remove reference to datetime_filesafe and show only datetime as a supported format field
README.md	Updated documentation examples and help text to use {datetime} instead of {datetime_filesafe}
test/duct_main/test_log_paths.py	Updated test to use new {datetime} format field instead of {datetime_filesafe}

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.19%. Comparing base (f35633e) to head (ebe4aac).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #375      +/-   ##
==========================================
- Coverage   91.38%   91.19%   -0.19%     
==========================================
  Files           8        8              
  Lines        1056     1056              
  Branches      138      138              
==========================================
- Hits          965      963       -2     
- Misses         69       70       +1     
- Partials       22       23       +1     

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