feat(ms-ai-architect): Sesjon 19 — B3 merge-apply (cross-skill + taksonomi-persist) [skip-docs]

Merge-apply = den tyngste destruktive op-en: eksekver en operatør-godkjent
merge_skills-entry → faktisk cross-skill skills/-mutasjon. Tre mekanismer
som sanitize/retire ikke trengte: kuratert relokasjon (ikke arkiv),
taksonomi-persistering, retire-av-absorbert.

- taxonomy.mjs (ny persist): saveTaxonomy (atomisk .tmp+rename, speiler
  saveDecisions) + ren applyCategoryReassignments (repointer KUN der eier
  === from; absent/allerede-flyttet = no-op). Lag-0 er normalt read-only;
  ENESTE writer = gated merge-apply.
- applyApprovedAction merge-gren (skill-ops.mjs): re-plan fersk → revalidér
  (kollisjon → guardrail.ok=false → abort) → flytt absorbed-refs → absorber
  (moveFile; flyttingen ER bevaringen set-equality beviser) → persister
  taksonomi-repoint fra FERSK plan (drift-sikker) → arkivér absorbed-SKILL.md
  FØR rmdir → flipp ledger approved→applied. Description-forsoning forblir
  MANUELL (planner-kontrakt: apply rører aldri absorber-SKILL.md).
  archiveMove→moveFile (generisk: arkiv + kuratert flytt).
- CLI apply-skill-op.mjs merge <absorber> <absorbed> — lookup via actionKey
  (merge_skills:<sortert par>); default PREVIEW, --apply = dobbel-gate.

TDD, tmpdir-fixtures: 0 ekte skills/-mutasjon. Ny test-skill-ops-merge-apply
(7) + 2 taksonomi-tester; fjernet foreldet merge-throws fra S18-fila. Tester:
kb-eval 78→84, kb-update 137→139; validate 239 · kb-integrity 192/192 uendret.
eval --json deterministisk uendret (K10 eng+infra FAIL, øvrige PASS).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/kb-update/lib/taxonomy.mjs

@@ -4,7 +4,7 @@
 // skill/category owns content, and update priority. poll-sitemaps,
 // discover-new-urls and report-changes READ this instead of embedding copies.
 
-import { readFileSync } from 'node:fs';
+import { readFileSync, writeFileSync, renameSync, existsSync, mkdirSync } from 'node:fs';
 import { join, dirname } from 'node:path';
 import { fileURLToPath } from 'node:url';
 
@@ -21,6 +21,41 @@ export function loadTaxonomy(dataDir = DEFAULT_DATA_DIR) {
   return JSON.parse(readFileSync(path, 'utf8'));
 }
 
+/**
+ * Save the domain taxonomy atomically (write to .tmp, then rename) — mirrors
+ * saveDecisions. The taxonomy is lag-0 truth and is normally read-only; the ONE
+ * writer is the gated merge-apply path (Sesjon 19), which repoints category
+ * ownership from an absorbed skill to its absorber. Detection scripts must NOT
+ * import this.
+ * @param {object} tax — taxonomy object to persist
+ * @param {string} [dataDir]
+ */
+export function saveTaxonomy(tax, dataDir = DEFAULT_DATA_DIR) {
+  if (!existsSync(dataDir)) mkdirSync(dataDir, { recursive: true });
+  const path = join(dataDir, 'domain-taxonomy.json');
+  const tmp = path + '.tmp';
+  writeFileSync(tmp, JSON.stringify(tax, null, 2) + '\n', 'utf8');
+  renameSync(tmp, path);
+}
+
+/**
+ * Repoint category ownership for a skill merge. Pure — returns a new taxonomy,
+ * mutates nothing. For each { category, from, to }, the category's owner is
+ * changed to `to` ONLY when it currently equals `from` (defensive: a category
+ * already repointed, or absent, is a no-op — never clobbered). All other fields
+ * survive untouched.
+ * @param {object} tax
+ * @param {Array<{category: string, from: string, to: string}>} reassignments
+ * @returns {object} new taxonomy
+ */
+export function applyCategoryReassignments(tax, reassignments) {
+  const next = { ...tax.category_skill };
+  for (const { category, from, to } of reassignments) {
+    if (next[category] === from) next[category] = to;
+  }
+  return { ...tax, category_skill: next };
+}
+
 /**
  * Target sitemap child-name prefixes to scan/poll.
  * @param {object} tax