feat: add a Blackbaud CRM (Enterprise) 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 Blackbaud CRM (the enterprise product built on the BBEC/SKY platform), which is a different product with a different data model from Blackbaud's Raiser's Edge / RE NXT already supported here — do not assume the two share field names or vocabulary. Blackbaud CRM models a gift as a "Revenue" record against a "Constituent", not a Raiser's Edge-style Gift record; confirm the actual shape from the docs before writing any alias.

Where to look

File	Purpose
philanthropy/ingest/_raisers_edge.py	Closest existing example of the pattern (same vendor, different product — useful for structure, not for field names)
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_blackbaud_crm.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/_blackbaud_crm.py: a header-alias map from Blackbaud CRM's real export labels (Revenue query/export) and SKY API field names onto the canonical contact_id / receive_date / total_amount / gift_type names; a documented default exclusion parameter for whatever this product's pledge-vs-payment split actually is; a blackbaud_crm_revenue_to_features(...) function (name it for what the product actually calls the record once you've checked); and a read_blackbaud_crm_revenue(path) reader delegating to read_civicrm_contributions.
2. Verify every field name and every commitment/payment vocabulary term against Blackbaud's own public SKY API / Blackbaud CRM 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, and do not carry over any Raiser's Edge field name without checking it's actually the same in this product.
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_blackbaud_crm.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 Blackbaud's own public docs for this specific product only — never from an institution's actual configuration or data, and never assumed from Raiser's Edge.
• 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 blackbaud_crm_revenue_to_features, read_blackbaud_crm_revenue; print('OK')"
make ci
make riskcov

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