Reconcile the data-ingest source index with the importers that now exist

_docs/runbooks/data-ingest.md's §2 source index and §4 still carry prose that
its own §12.1 table contradicts. One cause, so one issue:

• §2 rows 19/20 and the line at ~128, "Sources 19-21 have no ingest at all".
  §12.1's closed items 7 and 8 record that sponsors and testimonials both got
  importers (scripts/prod/import_sponsors.py,
  scripts/prod/import_testimonials.py, reading temporary/content/), and that
  the seeding migration and core/sponsor_history.py's hardcoded
  FEATURED_SUPPORTERS were deleted.
• A malformed §12.7 reference at line 423 — pre-existing and byte-identical
  at the base commit, found while verifying #321.

The document's whole premise is that an operator can act on it without asking
anyone a question, so a source index that says "no ingest at all" about a source
with a working importer is the expensive kind of wrong: it invites someone to
build a second one.

Scope: reconcile §2 and §4 against §12.1's closed entries, and fix the malformed
reference. Verify each claim against the code before rewriting it rather than
trusting either the prose or the §12.1 row.

Explicit non-goal: §12/§12.1 themselves (#321 re-checked them on 2026-09-07) and
the §6.1/§8 pin echoes (#327).

Refs #310

Start with _docs/runbooks/data-ingest.md, especially §§2, 4, 12.1, and the malformed §12.7 reference. Verify the relevant claims against scripts/prod/import_sponsors.py, scripts/prod/import_testimonials.py, temporary/content/, and core/sponsor_history.py, then reconcile §§2 and 4 without changing the explicit non-goals. Done means the source index matches the existing importers and the reference is valid.