feat(voyage): emit MANIFEST_PROFILE_DRIFT warning in plan-validator strict mode — brief assumption 7

Step 20 of v4.1 — implements drift detection in plan-validator.mjs per
brief Assumptions block 7: "Mismatch (e.g. korrupt manuell endring)
emitterer MANIFEST_PROFILE_DRIFT-warning fra plan-validator i --strict-modus."

Logic (after validateAllManifests in validatePlanContent):
  1. Strict-mode only — soft mode never emits drift warnings.
  2. Plan frontmatter must declare 'profile: <name>' to establish baseline.
  3. For each step manifest, if profile_used is set AND differs from plan
     profile, emit warning (NOT error) with code MANIFEST_PROFILE_DRIFT
     and location 'step N: profile_used = X, plan profile = Y'.

Forward-compat preserved: drift is a warning, plan remains valid:true.
Operators see the drift in --strict mode without parsing breaking.

New files:
  tests/validators/plan-validator-profile-drift.test.mjs — 4 tests
  tests/fixtures/plan-profile-drift.md                   — drift fixture

Tests verify:
  1. drift detected in strict mode → MANIFEST_PROFILE_DRIFT in warnings
  2. drift NOT detected in soft mode → strict gate honored
  3. matching profile → no drift warning
  4. no plan-level profile → drift detection silent (no baseline)

Tests: 479 pass + 2 skipped (Docker not installed).

plugins/voyage/tests/fixtures/plan-profile-drift.md

@@ -0,0 +1,57 @@
+---
+plan_version: "1.7"
+profile: economy
+phase_models:
+  - phase: brief
+    model: sonnet
+  - phase: research
+    model: sonnet
+  - phase: plan
+    model: sonnet
+  - phase: execute
+    model: sonnet
+  - phase: review
+    model: sonnet
+  - phase: continue
+    model: sonnet
+---
+
+# Test plan — profile drift fixture
+
+Frontmatter declares `profile: economy`. Step 1 manifest has matching
+profile_used. Step 2 manifest declares `profile_used: premium` — the
+drift case Step 20 of v4.1 plan-validator must catch in --strict mode.
+
+## Implementation Plan
+
+### Step 1: matching profile
+
+- Files: a.ts
+- Manifest:
+  ```yaml
+  manifest:
+    expected_paths:
+      - a.ts
+    min_file_count: 1
+    commit_message_pattern: "^feat:"
+    bash_syntax_check: []
+    forbidden_paths: []
+    must_contain: []
+    profile_used: economy
+  ```
+
+### Step 2: drift to premium
+
+- Files: b.ts
+- Manifest:
+  ```yaml
+  manifest:
+    expected_paths:
+      - b.ts
+    min_file_count: 1
+    commit_message_pattern: "^feat:"
+    bash_syntax_check: []
+    forbidden_paths: []
+    must_contain: []
+    profile_used: premium
+  ```