Close first librarian promote round-trip + add --accept flag

scripts/librarian_promote.py gains a --accept flag that copies the
generated draft to the suggested docs/research/ target in the same
invocation. This is explicit per-invocation opt-in, not auto-apply:
the operator types --accept knowingly, the cp is still a deliberate
acceptance decision, just expressed in one command instead of two.

docs/research/option-a-bench-result.md is the first promoted note,
generated by gemini from
docs/archive/option-a-bench-result-2026-05-07.md and accepted
verbatim. Spot-check verified that cited file:line locations match
current source (traversal.pyx:473-475 and
inference_server.py:226-228 carry the cited code), the Last-verified
header reflects today's date and HEAD, and the durable conclusion
(sync-blocking policy boundary as the structural ceiling, not IPC
plumbing) is preserved.

This closes the end-to-end loop the librarian was designed for:
oversize check surfaced docs/performance.md, the routing pass split
durable analysis into the archive entry, the promote dispatcher
turned the archive entry into a research draft, and the human
accept step landed it as a tracked research note. Took one LLM call.

Removes the now-resolved ignore-list entry for
docs/research/option-a-bench-result.md (the file exists; the
forward reference is real).

scripts/librarian.sh exits 0 against the working tree.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/librarian-ignore.txt

@@ -18,8 +18,3 @@ scripts/run_model_size_experiment.sh
 
 # Future config described in docs/plans/torch_compile.md
 configs/deep_cfr/default_compile.yaml
-
-# Hypothetical accept target named in docs/plans/librarian.md as a
-# smoke-test illustration; resolves naturally if Stage 2 promote is
-# accepted, otherwise stays a forward reference.
-docs/research/option-a-bench-result.md