docs(backlog): drop model-prescriptive session-protocol line

Session protocol shouldn't dictate which model to use — WPs (including the new
WP-53/54) are written to be executable regardless of model.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/project/backlog/README.md

@@ -9,7 +9,6 @@ This backlog **supersedes `docs/project/SHOWCASE-ROADMAP.md`**.
 
 ## Session protocol
 
-- **Switch to Opus first** (`/model opus`) before tackling any WP.
 - **One WP per session.** Read `CLAUDE.md`, this README, the WP file, and the WP's
   "Read first" list — then execute. Do not start the next WP in the same session.
 - The **Decisions** block in each WP is pre-made — don't relitigate it.