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/trekcontinue.md

@@ -293,6 +293,36 @@ if the env var is set; silently skip otherwise.
 {"ts":"<iso-8601>","project":"<project>","next_session_label":"<label>","status":"<status>"}
 ```
 
+## Profile (v4.1) — inheritance from plan-frontmatter
+
+Accepts `--profile <name>` where `<name>` is `economy`, `balanced`, `premium`,
+or a custom profile under `voyage-profiles/`. Default: `premium`.
+
+Unlike the other voyage commands, `/trekcontinue` defaults to **inheritance**
+from the resumed plan's frontmatter `profile:` field rather than to `premium`.
+This keeps a multi-session run consistent across resumptions.
+
+Resolution order (per `lib/profiles/resolver.mjs` `resolveTrekcontinueProfile`):
+1. `--profile` flag (source: `flag`) — explicit override, emits stderr advisory
+   `[voyage] profile inheritance overridden by --profile flag: <plan-profile> → <flag-profile>`
+2. `VOYAGE_PROFILE` env-var (source: `env`)
+3. Plan-frontmatter `profile:` field (source: `inheritance`) — typical case
+4. `premium` default for v4.0-style plans without `profile:` (source: `default`)
+
+The inherited profile drives `phase_models.continue` for the next session's
+orchestration. Operators who explicitly want to switch profiles mid-run pass
+`--profile <name>` and accept the advisory (e.g. drop from `premium` to
+`balanced` mid-run to save cost on later sessions).
+
+Examples:
+```
+/trekcontinue                                  # inherits from plan.md
+/trekcontinue --profile balanced               # explicit override + advisory
+VOYAGE_PROFILE=economy /trekcontinue           # env-var override
+```
+
+Stats records emit `profile` and `profile_source` per Phase 5 record.
+
 ## Hard rules
 
 - **Idempotent.** Running `/trekcontinue` twice in the same Claude session