Regenerate the retrieval baseline on demand, in CI

[mkreyman]

Unblocks the next two items in the agreed order.

Why it's needed

Adding a golden question — including one grown from real logged queries — makes the eval run
:incomparable. question_set_changed?/1 compares the question-id set and refuses,
rather than letting an unmatched question read as a non-regression. That guard is right; it
just means the baseline has to be regenerable, and it wasn't.

Why in CI — and a correction

I wrote in #670's body, and repeated it, that a local run "degrades to keyword_only and
would overwrite the embeddings-arm baseline." That was wrong. The eval's vectors are
deterministic functions of the committed text — no provider is involved — and the
no_embedding_key warnings in the log are the keyword_only arm passing
embedding: {:error, :no_api_key} deliberately, exactly as its moduledoc says.

The real reason is the corpus. This job seeds a fresh database; a developer's dev DB
carries whatever else it has accumulated. Same commit, same command:

	mrr	answered@5
CI	0.746	22 / 26
local	0.192	5 / 26

A baseline captured locally would bake one machine's private database into a deploy gate for
everyone.

Shape

• workflow_dispatch input, off by default.
• Uploads the file as an artifact; CI never commits it. A gate that can rewrite its own
  threshold unattended is not a gate — download, read the diff, commit deliberately.
• The gate step is skipped on a regeneration run. Leaving it in would compare the run
  against a baseline that same run had just written — a comparison that passes whatever the
  numbers are, which is the most expensive kind of green.

gh workflow run CI --ref <branch> -f regenerate_retrieval_baseline=true
gh run download <run-id> -n retrieval-baseline

Procedure documented in the runbook, including the measured local-vs-CI gap so the next
person doesn't re-derive it from a confusing local result the way I did.

mix precommit green (7,514). Reviewed inline (this session's system prompt forbids
dispatching review agents; per CLAUDE.md the gate is satisfied by the best available reviewer
with that stated).