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.

tests/commands/trekreview.test.mjs

@@ -13,9 +13,11 @@ import { parseDocument } from '../../lib/util/frontmatter.mjs';
 const HERE = dirname(fileURLToPath(import.meta.url));
 const ROOT = join(HERE, '..', '..');
 const COMMAND_FILE = join(ROOT, 'commands', 'trekreview.md');
+const MODES_DOC = join(ROOT, 'docs', 'command-modes.md');
 const PHASE = 'review';
 
 function read() { return readFileSync(COMMAND_FILE, 'utf8'); }
+function readModes() { return readFileSync(MODES_DOC, 'utf8'); }
 function readFixture(name) { return readFileSync(join(ROOT, 'tests', 'fixtures', name), 'utf8'); }
 function frontmatterOf(text) {
   const doc = parseDocument(text);
@@ -72,3 +74,50 @@ test('trekreview — SC7: brief_version 2.1 + no phase_signals + no partial →
     `sequencing gate must fire; errors=${JSON.stringify(r.errors)}`,
   );
 });
+
+// --- S11 (NW2 part B) — opt-in --workflow substrate routing ---
+
+test('trekreview — Phase 1 flag table documents the --workflow opt-in flag', () => {
+  const text = read();
+  assert.match(text, /--workflow/,
+    'commands/trekreview.md must document the --workflow opt-in flag (NW2 part B)');
+});
+
+test('trekreview — --workflow routes Phase 5–6 to the validated NW2 Workflow port script', () => {
+  const text = read();
+  assert.ok(text.includes('scripts/trekreview-armB.workflow.mjs'),
+    'the --workflow route must reference the bake-off-validated NW2 Workflow port script');
+});
+
+test('trekreview — Workflow path is opt-in; default stays prose (portability floor)', () => {
+  const text = read();
+  assert.match(text, /opt-in/i, '--workflow must be described as opt-in');
+  assert.ok(text.includes('Default stays prose'),
+    'the command must state the default Phase 5–6 path stays prose (not Workflow)');
+});
+
+test('trekreview — Workflow opt-in documents the Claude Code 2.1.154+ floor', () => {
+  const text = read();
+  assert.match(text, /2\.1\.154/,
+    'the --workflow path raises the consumer floor and must document Claude Code 2.1.154+');
+});
+
+test('trekreview — bake-off evidence (fidelity-equivalent / T2 results) is cited for the opt-in', () => {
+  const text = read();
+  assert.ok(
+    text.includes('T2-bakeoff-results.md') && /fidelity-equivalent/i.test(text),
+    'the opt-in must cite the S10 bake-off (docs/T2-bakeoff-results.md) fidelity-equivalence verdict',
+  );
+});
+
+test('command-modes.md — /trekreview table documents --workflow + its CC floor', () => {
+  const text = readModes();
+  const start = text.indexOf('## /trekreview modes');
+  assert.ok(start >= 0, 'command-modes.md missing "## /trekreview modes" section');
+  const end = text.indexOf('\n## ', start + 1);
+  const section = text.slice(start, end === -1 ? undefined : end);
+  assert.match(section, /--workflow/,
+    'command-modes.md /trekreview table must list the --workflow opt-in flag');
+  assert.match(section, /2\.1\.154/,
+    'command-modes.md /trekreview --workflow row must note the Claude Code 2.1.154+ floor');
+});