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

plugins/voyage/commands/trekexecute.md

@@ -1519,6 +1519,37 @@ VOYAGE_PROFILE=balanced /trekexecute --project ...
 Stats records emit `profile`, `phase_models`, and `profile_source` per
 Phase 9 record.
 
+## 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 `/trekexecute` specifically: `effort == 'low'` activates `--gates open`
++ sequential-only execution (no worktree-isolated parallel waves — runs
+all sessions in a single foreground loop). `effort == 'standard'` (or
+absent) → no change (default execution strategy applies). High-effort
+behavior is deferred to v5.1.1 per brief Non-Goal.
+
+### Sequencing gate surface
+
+When `/trekexecute --project <dir>` is invoked, optionally run
+`brief-validator.mjs --soft --json` against `{dir}/brief.md`. 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
 
 1. **No AskUserQuestion for execution decisions.** All execution decisions come