feat: datetime/date comparisons for row_filters (#87)

[ababic]

Summary

Fixes #87: lt / lte / gt / gte (and eq / neq) can compare ISO-8601 / Postgres timestamp text via format = "datetime", or calendar dates via format = "date".

Example

[row_filters."public.listing_order"]
retain = [
  { column = "created", op = "gte", value = "2025-02-14", format = "datetime" },
  { column = "created", op = "is_null" },
]

Behavior

• Parses common pg_dump shapes (YYYY-MM-DD HH:MM:SS.fff+00, T separators, Z, short offsets)
• Date-only thresholds with format = "datetime" are midnight UTC
• Thresholds validated at config load (fail closed)
• Unparseable cell values do not match (fail closed)
• Numeric compares unchanged when format is omitted

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets --all-features (177 passed)
• CLI smoke: COPY retain window keeps rows on/after cutoff and NULLs; drops older rows

Tests / docs

• Unit tests for parsing, retain windows, date-only compares, and config validation
• README, configuration guide, .dumplingconf.example, CHANGELOG, AGENTS.md