feat(voyage): document --profile flag in all 6 commands — SC #4 + arv-policy

Step 7 av v4.1-execute (Wave 3, Session 4).

Legg ny "## Profile (v4.1)"-seksjon i hver kommando-fil rett før "## Hard rules":
- trekbrief.md: --profile + VOYAGE_PROFILE + premium default
- trekresearch.md: + economy/balanced auto-disable external_research_enabled
- trekplan.md: + plan.md frontmatter recording for inheritance
- trekexecute.md: + 4-step resolution (flag > env > inheritance > default)
- trekreview.md: + opus-default for review-deepening
- trekcontinue.md: spesiell — INHERITANCE er default (ikke premium), --profile
  overstyr emitter stderr-advarsel

Tester (13 nye, baseline 432 → 445):
- 6 commands × 2 (--profile + VOYAGE_PROFILE)
- trekcontinue.md "inheritance"-keyword

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/voyage/commands/trekbrief.md

@@ -675,6 +675,25 @@ Append one record to `${CLAUDE_PLUGIN_DATA}/trekbrief-stats.jsonl`:
 If `${CLAUDE_PLUGIN_DATA}` is not set or not writable, skip silently.
 Never let stats failures block the workflow.
 
+## Profile (v4.1)
+
+Accepts `--profile <name>` where `<name>` is one of `economy`, `balanced`,
+`premium`, or a custom profile under `voyage-profiles/`. Default: `premium`.
+
+Resolution order (per `lib/profiles/resolver.mjs`):
+1. `--profile` flag (source: `flag`)
+2. `VOYAGE_PROFILE` environment variable (source: `env`)
+3. `premium` default (source: `default`)
+
+Examples:
+```
+/trekbrief --profile economy
+VOYAGE_PROFILE=balanced /trekbrief
+```
+
+Stats records emit `profile`, `phase_models`, and `profile_source` so operators
+can audit which profile drove which session.
+
 ## Hard rules
 
 1. **Interactive only.** This command requires user input. There is no