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).
