FiscalYearGroupedSplitter: drop_repeat_donors defaults to False, so the default path still leaks a static per-donor label across folds

[shivamlalakiya]

What's wrong

FiscalYearGroupedSplitter(drop_repeat_donors=...) defaults to False. For a
static per-donor label (e.g. is_major_donor, the label used in the README,
the benchmarks page, and scripts/benchmark_models.py), leaving it at the
default lets the same donor's rows land in both the train and test fold of a
split. That is the exact leakage this splitter exists to prevent, and it is
live in the default path, not an opt-in footgun.

Where

File	Locate with
philanthropy/model_selection/_temporal_donor_splitter.py	grep -n "drop_repeat_donors: bool = False" philanthropy/model_selection/_temporal_donor_splitter.py

What to change

1. Add a DeprecationWarning in FiscalYearGroupedSplitter.split (or
   __init__) when drop_repeat_donors is left at its default False,
   stating that the default will become True in a future release and that
   callers relying on repeat donors appearing in both folds should pass
   drop_repeat_donors=False explicitly to silence it.
2. Update the class docstring's drop_repeat_donors parameter entry to state
   the planned default flip and the release it lands in, following the same
   pattern already used for WealthScreeningImputerKNN(group_col_idx=...)
   (see CHANGELOG.md, ## [Unreleased] section, Deprecated).
3. Add a ## [Unreleased] CHANGELOG.md entry and a CONTRIBUTORS.md line
   in the same PR (required by AGENTS.md).

Tests to add or extend

• File: tests/test_temporal_donor_splitter.py
• Add: test_default_drop_repeat_donors_emits_deprecation_warning
• Add: test_explicit_drop_repeat_donors_false_silences_warning

Done when

python -c "
import warnings
from philanthropy.model_selection import FiscalYearGroupedSplitter
with warnings.catch_warnings(record=True) as w:
    warnings.simplefilter('always')
    FiscalYearGroupedSplitter()
    assert any(issubclass(x.category, DeprecationWarning) for x in w)
"
make ci

First time here?

Read CONTRIBUTING.md and AGENTS.md.
Add a ## [Unreleased] CHANGELOG entry and yourself to CONTRIBUTORS.md in
the same PR.