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

plugins/voyage/commands/trekresearch.md

@@ -435,6 +435,36 @@ Stats records emit `profile`, `phase_models`, `parallel_agents`,
 `external_research_enabled`, 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 `/trekresearch` specifically: `effort == 'low'` activates the
+existing `--quick`-equivalent code-path (inline research, no agent swarm).
+`effort == 'standard'` (or absent) → no change. High-effort behavior is
+deferred to v5.1.1 per brief Non-Goal.
+
+### Sequencing gate surface
+
+When `/trekresearch --project <dir>` is invoked and `{dir}/brief.md`
+exists, optionally run `brief-validator.mjs --soft --json` against it.
+If `BRIEF_V51_MISSING_SIGNALS` appears 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;
+commands surface, don't re-enforce.
+
 ## Hard rules
 
 - **No planning:** This command produces research briefs, not implementation plans.