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

plugins/voyage/commands/trekplan.md

@@ -894,6 +894,36 @@ VOYAGE_PROFILE=balanced /trekplan --project ...
 Stats records emit `profile`, `phase_models`, `parallel_agents`, and
 `profile_source` so operators can audit which profile drove which session.
 
+## 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 `/trekplan` specifically: `effort == 'low'` activates the existing
+`--quick`-equivalent code-path (skip Phase 5 agent swarm — plan directly
+without exploration agents). `effort == 'standard'` (or absent) → no
+change. High-effort behavior is deferred to v5.1.1 per brief Non-Goal
+("No complete per-phase effort dictionary").
+
+### Sequencing gate surface
+
+Phase 1 already calls `brief-validator.mjs --soft`. If the validator
+returns `BRIEF_V51_MISSING_SIGNALS` in `errors` (brief_version ≥ 2.1
+without `phase_signals` or `phase_signals_partial: true`), halt with a
+one-line message: `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-driven**: Every plan decision must trace back to a section of the