feat: add a Little Green Light gift export reader (ingest preset)

[shivamlalakiya]

What's needed

Same pattern as the other CRM bridges in philanthropy.ingest (CiviCRM, Raiser's Edge, NPSP): a thin, CRM-specific header-alias map plus a commitment/payment exclusion filter, delegating the roll-up to the shared aggregator in philanthropy/ingest/_civicrm.py. This issue adds the bridge for Little Green Light.

Where to look

File	Purpose
philanthropy/ingest/_raisers_edge.py	Closest existing example: header aliasing, a documented default exclusion set, delegation to the shared aggregator
philanthropy/ingest/_civicrm.py	The shared aggregator (civicrm_contributions_to_features, read_civicrm_contributions) and its private helpers
tests/test_raisers_edge.py	The test depth expected of your new tests/test_little_green_light.py
philanthropy/cli.py	_FEATURE_SOURCES and the features --source dispatch — wire your new source in the same shape, unless the read_gifts preset registry (a separate, in-flight PR) has already landed, in which case register there instead

What to build

1. philanthropy/ingest/_little_green_light.py: a header-alias map from Little Green Light's real export labels (its gift/donation export) and API field names onto the canonical contact_id / receive_date / total_amount / gift_type names; a documented default exclusion parameter for whatever its pledge-vs-payment split actually is (find out exactly how from the docs — do not assume it matches Raiser's Edge or NPSP); a little_green_light_gifts_to_features(...) function; and a read_little_green_light_gifts(path) reader delegating to read_civicrm_contributions.
2. Verify every field name and every commitment/payment vocabulary term against Little Green Light's own public help center and API documentation before writing it down. Cite the doc URLs you actually used in your PR body, the way PR #233 does for NPSP. This issue does not assert any field name as fact — confirm everything yourself.
3. Export the new names from philanthropy/ingest/__init__.py (__all__, alphabetical) and add a stability-tier row to docs/reference/index.md (Tier 2/Beta — a test enforces every public symbol has a row).
4. tests/test_little_green_light.py at the depth of tests/test_raisers_edge.py: header aliasing from every spelling the docs name, the commitment/payment double-count trap and its exclusion, the exclusion set being documented/overridable/disabled by None, currency parsing, schema/dtypes, empty input, missing required columns.
5. CHANGELOG.md entry under ## [Unreleased], and add yourself to CONTRIBUTORS.md if you'd like.

Hard constraints (same as the rest of this ingest series)

• Dependencies stay scikit-learn/pandas/numpy/joblib only — no new package, no API client, no network code anywhere in philanthropy/. File-export reader only, never a live API connector.
• No new estimator class.
• Build the exclusion logic from Little Green Light's own public docs only — never from an institution's actual configuration or data.
• Follow AGENTS.md's order: implement, export, verify with python -c "...", then write tests, then make ci.

Verifying

python -m pip install -e ".[dev]"
python -c "from philanthropy.ingest import little_green_light_gifts_to_features, read_little_green_light_gifts; print('OK')"
make ci
make riskcov

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