feat: allow repeated --filter flags for exports

[jacalata]

Summary

--filter previously accepted a single value and required multi-pair filters to be joined with &, e.g. --filter "A=1&B=2". That parsing scheme prevents literal & inside a value (e.g. Product Name=AT&T 841000 Phone becomes Product Name=AT, then a bogus T 841000 Phone clause).

Make --filter repeatable via argparse action="append". Each flag now carries exactly one COLUMN=VALUE pair — no & splitting — so literal & in a value passes through untouched.

Back-compat

When only one --filter flag is present, the value is still split on & so existing scripts keep working:

--filter "Region=West&Product=Widget"     # still works
--filter "Region=West" --filter "Product=Widget"   # preferred
--filter "Product=AT&T"                    # new: repeated form allows '&' in value

Verification

End-to-end against a live workbook with product names containing &:

Filter	Before	After
--filter "Region=East" --filter "Product Name=AT&T 841000 Phone"	parse error	1 row (AT&T 841000 Phone)
--filter "Region=East&Product Name=Widget" (back-compat)	1 row	1 row

Test plan

• pytest tests/commands/test_datasources_and_workbooks_command.py tests/parsers/test_parser_export.py — 34 passed
• Manual e2e: repeated --filter with & in the value returns the expected row
• Manual e2e: single-flag &-joined form still parses and filters correctly

Follow-ups

Related work in flight:

• fix: preserve '=' inside --filter values #442 — preserve = inside --filter values

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