Finish source→origin migration for legacy snapshots

[Marigold]

Background

OWID ETL is gradually replacing the legacy meta.source block in snapshot
DVCs with the modern meta.origin block. The migration has been running
in waves driven by the migrate-source-to-origin skill (see
.claude/skills/migrate-source-to-origin/SKILL.md), with the skill itself
being tightened over time as we learned where it produces poor output.

Work in progress

Two open PRs cover the bulk of the remaining migrations:

• Migrate Source → Origin for dag/migrated.yml snapshots #5978 — data-migrate-defra-air: covers the snapshots referenced
  by dag/migrated.yml (the legacy-OWID-datasets bucket). Includes
  re-migrations of files where earlier skill versions had paraphrased
  descriptions, fabricated content for empty source descriptions, left
  HTML in origin fields, or used Various sources where producers were
  in fact named.
• 📊 Migrate excess_mortality snapshots to meta.origin #6027 — data-migrate-complex-sourceorigin: targets the
  trickier compilations and multi-product database slices that the
  skill flagged as needing closer attention.

Manual review is required

The migrations are AI-generated and the skill, while better than it
was, is not perfect. Before either PR is merged, every changed
DVC needs a human pass to check:

• Producer: matches the actual data publisher (institution or
  authors), not OWID's internal label, and not generic Various sources when ≤3 producers are named in the legacy.
• Title vs title_snapshot: STEP 1 of the skill ("does the data
  product coincide with this snapshot?") is a judgment call; the agent
  often picks coincide when the legacy implies a multi-product
  database slice (and vice versa).
• description is verbatim from the legacy, not paraphrased,
  fabricated, or rewritten.
• URLs preserved: legacy source.url prose blobs must be
  extracted into url_main/url_download, not silently dropped or
  left as raw HTML in description.
• citation_full: producer's preferred citation, period at end,
  no curly quotes, no &.
• date_accessed: not the migration tool's "today's date"
  default — should match the snapshot's intent.

What's left after these PRs

Once #5978 and #6027 land, the remaining files with legacy
meta.source blocks (mostly active-but-untouched snapshots outside
dag/migrated.yml) will need their own pass — likely incremental,
file-by-file as snapshots get touched for unrelated reasons.