feat(voyage): add lib/profiles/resolver.mjs — locked interface SC #5-#9

Step 6 av v4.1-execute (Wave 2, Session 2).

Implementer locked interface contract fra brief Preferences:

- loadProfile(name, opts) → ProfileObject
  Leser lib/profiles/<name>.yaml (built-in) eller custom fra
  <cwd>/voyage-profiles/ > ~/.claude/voyage-profiles/. Throws Error med
  cause: PROFILE_NOT_FOUND. Returnerer parsed object med phase_models
  flattened til {brief: 'sonnet', research: 'opus', ...} (object form
  for downstream JSON-stats).

- resolveProfile(argv, env) → {profile, profile_source}
  Ordre: --profile flag > VOYAGE_PROFILE env > 'premium' default.

- resolveTrekcontinueProfile(planPath, argv, opts) → {profile, profile_source}
  --profile flag wins ('flag'); ellers leser plan.md frontmatter
  ('inheritance'); v4.0-stil plan uten profile-felt → 'default' premium
  (backward-compat). Flag overstyrer arv → console.error advisory.

- validateProfileFile(path) → Result
  Tynn re-eksport av validateProfile fra profile-validator.mjs.

- findProfilePath(name, opts) → {path, attempted}
  Lookup-helper. attempted-array brukes i error-melding for HIGH-risk-
  mitigering (ENOENT-diagnose).

Tester (13 nye, baseline 387 → 400):
- SC #5 x4 (loadProfile economy/balanced/premium + PROFILE_NOT_FOUND)
- SC #6 (flag > env > default ordre)
- SC #7 (performance: 1000-iter < 50ms gjennomsnitt; faktisk ~0.055ms)
- SC #8 x2 (cwd > home precedence + error-msg attempted-paths)
- SC #9 x2 (inheritance + flag-override-advisory)
- Backward-compat x2 (v4.0 plan + non-existent plan)
- validateProfileFile re-export sanity

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

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

@@ -0,0 +1,26 @@
+---
+plan_version: "1.7"
+profile: balanced
+phase_models:
+  - phase: brief
+    model: sonnet
+  - phase: research
+    model: sonnet
+  - phase: plan
+    model: opus
+  - phase: execute
+    model: sonnet
+  - phase: review
+    model: opus
+  - phase: continue
+    model: sonnet
+---
+
+# Test plan (with profile)
+
+This fixture has explicit profile + phase_models in frontmatter.
+
+## Implementation Plan
+
+### Step 1: Stub
+- Files: src/stub.mjs

plugins/voyage/tests/fixtures/plan-without-profile.md

@@ -0,0 +1,15 @@
+---
+plan_version: "1.6"
+---
+
+# Test plan (v4.0-style, no profile field)
+
+This fixture is a v4.0-style plan WITHOUT the v4.1 profile/phase_models fields.
+Used by tests/lib/profile-application.test.mjs to verify backward-compat
+edge-case: resolveTrekcontinueProfile returns {profile: 'premium', profile_source: 'default'}
+without throwing when the plan has no profile concept.
+
+## Implementation Plan
+
+### Step 1: Stub
+- Files: src/stub.mjs