feat(voyage): wire 4 downstream commands to brief.phase_signals + composition rule (v5.1)

plugins/voyage/commands/trekreview.md

@@ -357,6 +357,36 @@ VOYAGE_PROFILE=premium /trekreview --project ...
 
 Stats records emit `profile` and `profile_source`.
 
+## Composition rule (v5.1)
+
+Independent of the profile system. When `brief.md` carries
+`phase_signals` (brief_version ≥ 2.1), each downstream phase resolves
+effort + model as:
+
+```
+effort_for_phase = brief.phase_signals[<phase>]?.effort ?? 'standard'
+model_for_phase  = brief.phase_signals[<phase>]?.model  ?? profile.phase_models[<phase>]
+```
+
+The brief signal wins per-phase when present; the profile fills any
+gaps. There is no helper module — composition is documented prose in
+each downstream command.
+
+For `/trekreview` specifically: `effort == 'low'` activates the existing
+`--quick`-equivalent code-path (skip the brief-conformance reviewer; run
+correctness-only). `effort == 'standard'` (or absent) → no change.
+High-effort behavior is deferred to v5.1.1 per brief Non-Goal.
+
+### Sequencing gate surface
+
+Phase 1 already calls `brief-validator.mjs --soft` against `{brief_path}`.
+If the validator returns `BRIEF_V51_MISSING_SIGNALS` in `errors`
+(brief_version ≥ 2.1 without `phase_signals` or `phase_signals_partial:
+true`), halt with: `Brief is brief_version 2.1 but does not carry
+phase_signals — re-run /trekbrief to commit them (Phase 3.5).`
+Enforcement is validator-only; this surface just makes the friendly hint
+readable.
+
 ## Hard rules
 
 - **Brief is the contract.** Every finding in the review traces to a