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.

lib/parsers/arg-parser.mjs

@@ -27,7 +27,7 @@ const FLAG_SCHEMA = {
     aliases: {},
   },
   trekreview: {
-    boolean: ['--quick', '--fg', '--dry-run', '--validate'],
+    boolean: ['--quick', '--fg', '--dry-run', '--validate', '--workflow'],
     valued: ['--project', '--since', '--profile'],
     aliases: {},
   },