feat(pair-corpus-honesty): add vibe-py, vfc-js and agent-vfc slices with mechanism vocabulary

Tasks 1.1, 5.1, 5.2, 5.4 and the data half of 5.3.

- mechanisms.toml: closed vocabulary v1 (20 ids) every expected row cites.
- vibe-py: 35 Real-Vuln-Benchmark pairs (Apache-2.0), human provenance only
  because the LLM-generated repositories carry no license (escalated).
- vfc-js: 17 Node fix-commit pairs harvested upstream (MIT/Apache-2.0),
  SecBench.js itself is not vendored (no license); sinks derived on 13/17.
- agent-vfc: 29 agent-trailer fixes as catalog-candidates.toml plus
  REVIEW.md; the loaded catalog stays empty until a human reviews them.
- datasets.toml and README document every slice and its review status.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

benchmarks/pairs/vfc-js/diskusage-ng-0-2-6/fixed.js

@@ -0,0 +1,26 @@
+// Provenance: iximiuz/node-diskusage-ng  (fixed).
+// repo: iximiuz/node-diskusage-ng
+// commit: 48e7e093486b528f0c81ec699573e0e4a431b8d3
+// parent: 53c505506553c83861d1bd67823d6a382d3a5a43
+// commit_url: https://github.com/iximiuz/node-diskusage-ng/commit/48e7e093486b528f0c81ec699573e0e4a431b8d3
+// cve: CVE-2020-7631
+// license: MIT
+// function: diskusage
+// relpath: lib/posix.js
+// provenance: human
+// mechanism: source_reaches_sink
+// upstream_start: 6
+
+function diskusage(path, cb) {
+    execFile('df', ['-k', path], function(err, stdout) {
+        if (err) {
+            return cb(err);
+        }
+
+        try {
+            cb(null, parse(stdout));
+        } catch (e) {
+            cb(e);
+        }
+    });
+}