007 T1: pin the reviewer's model to claude-opus-5 nf-tw2v ← Beads

Twinned from https://github.com/cfsh/ninefold/issues/878 by tools/beads/import_github.py's reconcile pass.

Implement T1 from design_docs/007-reviewer-verdicts-for-readers.md (§8, decision D1). Split out of the umbrella #872, per that issue's own note that the session taking it may split into per-task issues.

Goal: the reviewer runs claude-opus-5 because we chose it, not because the CLI defaults to it.

• dispatch.py builds the review invocation with no --model flag (Dispatcher.spawn(), near line 1081), so it runs whatever the reviewer box's claude CLI defaults to. Design 007 D1 decided to pin it explicitly.
• Add "--model", "claude-opus-5" to the claude -p argv, with a one-line comment citing design 007 D1.
• Tests: whichever tools/tests/ module covers dispatch argv construction; if none asserts the argv, add a unit test that builds the spawn argv and asserts --model claude-opus-5 is present.

PR shape: one small, mechanical PR. Done when: merged and a subsequent real review's log records claude-opus-5 as an explicit choice.

Independent of T2–T5; land any time.