fix(newos4): unmeasured skills are untracked, world-model writes verified, skill audit

External review packet (os4-external-review-v4.6.51.html) triaged against the code:
2 findings confirmed and fixed, 4 could not be reproduced and were recorded as such.

- module-memory-graph.js: getStaleSkills() treated a missing lastUsed as 999 days
  unused, so skill-audit recommended archiving every skill (22 of 22). Skills with no
  usage telemetry are now collected by getUntrackedSkills() and never counted stale.
  Live: "Stale: 0" + "Untracked: 22 (no usage telemetry — NOT stale)".
- module-world-model.js: _saveToVFS() ignored fsWriteJSON()'s return value and promoted
  whatever a truthy read returned. Now write -> read-back -> byte compare -> promote,
  with every return checked. Regression test modules/test/test-world-model-save.js
  (red on the old code: promoted=true with errs=[]; green now).
- scripts/newos4-skill-audit.py: 5 mechanical probes (version x3, read_file paging +
  context budget, selection symbols, required scripts, extKeys) + skill presence;
  exits non-zero on drift. 7/7 pass today. Weekly job cb6bc2a16216 runs it Mondays.

Not reproduced (recorded in admin/docs/newos4-external-review-verification-2026-10-23.md):
world_model_query empty-payload throw (live call returns JSON), pipes-as-command-names,
ls skills empty (23 entries via fs.readDir), ~66 agent dirs (4 live).

1 files changed, 12 insertions, 0 deletions

diff --git a/modules/core/module-memory-graph.js b/modules/core/module-memory-graph.js
index ffc79e0..093d2e4 100644
--- a/modules/core/module-memory-graph.js
+++ b/modules/core/module-memory-graph.js
@@ -469,9 +469,16 @@ class MemoryGraphModule {
             // v4.5.90b fix: Use configurable _decayDays (P1 from review)
             const staleThreshold = this._decayDays * 24 * 60 * 60 * 1000;
             var stale = [];
+            var untracked = [];
             for (var name of Object.keys(index)) {
                 var stats = index[name] || {};
                 if (stats.immune) continue;
+                // A skill with no usage telemetry has never been measured. Calling it stale is how
+                // the audit came to recommend archiving the system's entire procedural memory.
+                if (stats.lastUsed === undefined && stats.useCount === undefined) {
+                    untracked.push(name);
+                    continue;
+                }
                 var useCount = stats.useCount || 0;
                 var lastUsed = stats.lastUsed ? new Date(stats.lastUsed).getTime() : 0;
                 if (useCount === 0 && (now - lastUsed > staleThreshold || lastUsed === 0)) {
@@ -479,12 +486,17 @@ class MemoryGraphModule {
                     stale.push({ name: name, daysUnused: daysUnused });
                 }
             }
+            this._untrackedSkills = untracked;
             return stale;
         } catch (e) {
             return [];
         }
     }
 
+    getUntrackedSkills() {
+        return Array.isArray(this._untrackedSkills) ? this._untrackedSkills : [];
+    }
+
     // v4.5.89: Configure decay policy
     decayConfig(args) {
         if (args.length < 2) {