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.

commands/trekreview.md

@@ -52,6 +52,7 @@ FLAG_SCHEMA `trekreview` entry):
 | `--validate` | boolean | Schema-only check on existing `{project_dir}/review.md`. No LLM calls. |
 | `--dry-run` | boolean | Print the discovered scope and triage map. Skip writes. |
 | `--fg` | boolean | No-op alias (foreground is default). |
+| `--workflow` | boolean | **(opt-in, NW2)** Run Phase 5–6 on the bake-off-validated Workflow substrate (`scripts/trekreview-armB.workflow.mjs`) instead of the default prose Agent-tool path. Requires **Claude Code 2.1.154+**. Combines with `--quick`. See *§ Phase 5–6 via the Workflow substrate*. |
 
 Resolution:
 1. If `--project` is missing, print usage and stop:
@@ -75,6 +76,13 @@ Set `mode`:
 - `quick` if `--quick` is set.
 - `default` otherwise.
 
+Set `workflow_substrate` (orthogonal to `mode` — a substrate choice, not a behavior mode):
+- `true` if `--workflow` is set — Phase 5–6 run on the Workflow substrate (see the Phase 5
+  routing gate). The Workflow tool requires **Claude Code 2.1.154+**; if it is unavailable,
+  fall back to the prose path and note the fallback in the Executive Summary.
+- `false` otherwise. **Default stays prose**: the substrate is opt-in, so the lower
+  portability floor of the prose path is preserved unless the operator opts in.
+
 ## Phase 2 — Validate brief
 
 Run the brief validator in soft mode — the brief is upstream context, not
@@ -181,6 +189,12 @@ If `mode == dry-run`: print the triage map and exit.
 
 ## Phase 5 — Launch parallel reviewers
 
+**Substrate routing (opt-in `--workflow`).** When `workflow_substrate == true`, run
+Phases 5–6 via the Workflow substrate documented in *§ Phase 5–6 via the Workflow
+substrate* (below Phase 6), then resume at Phase 7 with the returned
+`{verdict, findings}`. When `false` (the default), run the prose Agent-tool path
+described in the rest of this phase.
+
 Launch two reviewer agents **in parallel** via the Agent tool — one
 message, multiple tool calls.
 
@@ -240,6 +254,54 @@ The coordinator's output is the full review.md content — frontmatter +
 body sections + trailing JSON block. Do NOT re-run the reviewers based
 on the coordinator's output.
 
+## Phase 5–6 via the Workflow substrate (opt-in `--workflow`)
+
+Runs **only** when `workflow_substrate == true`. This is the **NW2 port**: it expresses
+the SAME Phase 5–6 pipeline (parallel reviewers → triplet-dedup → coordinator verdict) as
+a single Workflow, reusing the NW1 findings schema. The S10 bake-off found it
+**fidelity-equivalent** to the prose path — see `docs/T2-bakeoff-results.md` (verdict
+**POSITIVE**: verdict-match 1.0, issue-coverage 100%, `(file,rule_key)` jaccard ≥
+within-arm, tokens +4.4%). It stays **opt-in**, not the default, because the Workflow tool
+raises the consumer floor to **Claude Code 2.1.154+** (outward-facing; the prose path
+keeps the lower floor).
+
+Invoke the port via the **Workflow** tool with the Phase 1–4 output pinned into `args`:
+
+```
+Workflow({
+  scriptPath: "${CLAUDE_PLUGIN_ROOT}/scripts/trekreview-armB.workflow.mjs",
+  args: {
+    briefPath: "{brief_path}",
+    diffPath:  "{path to the unified diff file from Phase 3}",
+    triage:    "{triage map as 'path → treatment' lines from Phase 4}",
+    quick:     {true if mode == quick, else false}
+  }
+})
+```
+
+Contract (verified in S10 part B — follow exactly):
+- **Pass `args` as a JSON object.** The script defensively re-parses a JSON *string*, but
+  the object form is the contract.
+- **Reviewers are `StructuredOutput`-schema-forced** — `rule_key` is enum-enforced at the
+  tool layer (stronger than the prose path's post-hoc NW1 check), so there is no
+  `JSON.parse`/re-ask dance.
+- **Recover the result from the `RESULT_JSON:{…}` line** inside the workflow output logs.
+  The script returns `{verdict, findings, ...}` AND logs it as that line; the
+  notification's `<result>` may be truncated, so parse the logged line.
+- The reviewer/coordinator agentTypes are namespaced inside the script
+  (`voyage:brief-conformance-reviewer`, `voyage:code-correctness-reviewer`,
+  `voyage:review-coordinator`).
+
+Then continue at **Phase 7** exactly as the prose path does — Phase 7 rendering, Phase 8
+validation, and the operator gate are **shared** and substrate-independent (both paths
+return the same `{verdict, findings}` shape).
+
+**Known limitation (per bake-off §Posture, surfaced not hidden).** Classifier interference
+was measured **0** at 9-agent concurrency in the session's default permission mode; an
+explicit `auto`/`bypass`-mode re-run was not performed (the permission mode is operator-set,
+not settable from within a session). trekreview's small fan-out showed 0 interference in S8
+and S10. The large fan-out case (the trekplan swarm) is out of NW2 scope.
+
 ## Phase 7 — Write review.md
 
 Write the coordinator's output verbatim to: