TDD step 1 of the phase 1 plan: typed error hierarchy rooted in
IngestError, manifest parsing with schema validation before any source
call — id grammar, polymorphic source types, extraction rules, verdict
reservation (okf_type and filename namespace), and the no-embedded-
credentials rule for base_url. Dev tooling (pytest/mypy/ruff) added as
a PEP 735 dependency group; runtime dependencies stay empty.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QeqhJpYQyghASjiJo5EhGg
