Unify date-like validation for document created inputs

[NickChristensen]

We accept date-like values for the document created field (add/update), but validation is inconsistent. There are existing DateString/DateTimeString types plus the new dateLike flag/parse logic used by documents:list date filters.

Goal:

• Define a single date-like input strategy for created that reuses shared parsing/validation.
• Align add/update behavior and error messaging with the documents:list date-like flag.

Notes:

• Ensure we handle both YYYY-MM-DD and ISO 8601 datetimes consistently.
• Consider whether the shared logic should live in helpers/flags for reuse across commands.