feat(voyage): S11 — NW2 part B, integrate opt-in --workflow flag

Make the bake-off-validated Workflow substrate (Arm B) reachable behind an
opt-in --workflow flag for /trekreview. Default Phase 5-6 path stays prose
to preserve the lower portability floor; --workflow raises the consumer
floor to Claude Code 2.1.154+ (the Workflow tool).

- arg-parser: --workflow added to trekreview boolean flags
- commands/trekreview.md: flag row + Phase 5 substrate-routing gate +
  new section 'Phase 5-6 via the Workflow substrate' (invocation contract,
  S10 gotchas, bake-off citation, auto/bypass residual as Known limitation)
- docs/command-modes.md: --workflow row in /trekreview table
- routes to existing scripts/trekreview-armB.workflow.mjs (byte-identical to
  the S10 part-B POSITIVE build); integration is pure routing, no script change

TDD: 8 new tests (arg-parser flag recognition + combine; command/doc prose
pins for route, opt-in posture, 2.1.154+ floor, bake-off evidence).
Suite 662 -> 670 (668 pass / 2 skip / 0 fail). plugin validate clean modulo
known root-CLAUDE.md warning. Resolves W1-narrow-wins-plan.md S11.

docs/command-modes.md

@@ -72,6 +72,7 @@ If `{project_dir}/architecture/overview.md` exists (typically produced by an opt
 | `--validate` | Schema-only check on existing `{dir}/review.md`. No LLM calls |
 | `--dry-run` | Print discovered scope + triage map; skip writes |
 | `--fg` | No-op alias (foreground is default) |
+| `--workflow` | (opt-in, NW2) Run Phase 5–6 on the bake-off-validated Workflow substrate (`scripts/trekreview-armB.workflow.mjs`) instead of the default prose path. Default stays prose; requires **Claude Code 2.1.154+** (raises the consumer floor — opt-in for portability). Fidelity-equivalent per `docs/T2-bakeoff-results.md` |
 | `--profile <name>` | (v4.1.0) Model profile for the review phase. |
 
 ## /trekcontinue modes