fix(sources): audit walker inverted pruneDir — nested sources scanned 0 files (#2678)

The audit walk in `gbrain sources audit` used `if (pruneDir(entry, dir)) continue;`
but pruneDir() returns true = descend, false = prune (core/sync.ts). The
inversion made the walker skip every legitimate subdirectory (any source with
nested content reports 'Files scanned: 0 markdown files') while descending
into exactly the trees it should skip (node_modules/, .git/, vendor/, .raw/).

The walker's own comment says 'Mirror gbrain sync's descent rules' — this
makes it actually do so.

Repro on a real brain (v0.42.56/57): a comms source with per-contact
subdirectories audits 0 files; a flat source audits correctly.

Co-authored-by: Idrees Kamal <idreeskamal@MacBook-Air.local>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/commands/sources.ts

@@ -1147,7 +1147,8 @@ async function runAudit(engine: BrainEngine, args: string[]): Promise<void> {
         continue;
       }
       if (stat.isDirectory()) {
-        if (pruneDir(entry, dir)) continue;
+        // pruneDir returns true = descend, false = prune (see core/sync.ts).
+        if (!pruneDir(entry, dir)) continue;
         walk(full);
       } else if (entry.endsWith('.md')) {
         files.push(full);