docs: add PRP-28 forecast explainability driver attribution (#224)

[w7-mgfcode]

Adds the PRP for a new app/features/explainability/ vertical slice — rule-based, deterministic forecast explanations for the three baseline forecasters.

Closes #224.

Doc-only change: PRPs/PRP-28-forecast-explainability-driver-attribution.md. No code touched.

Derived from .agents/plans/forecast-explainability-driver-attribution.md, validated against the live codebase — notably the stale Alembic head was corrected (378c112e4b32 → 43e35957a248) and an alembic/env.py model-registration task added. PRP passed the prp-quality-agent gate (READY, 9/10).

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