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/README.md

@@ -0,0 +1,14 @@
+# vfc-js slice
+
+Server-side JavaScript function pairs harvested from the **upstream fix commits** that
+[SecBench.js](https://github.com/cristianstaicu/SecBench.js) records in its package metadata
+(`fixCommit`, `sink`). SecBench.js itself has no license file, so none of its files are vendored;
+each excerpt carries the upstream package's license.
+
+- Classes: command injection, path traversal, code injection first. Prototype pollution and ReDoS
+  rows may be added with their own mechanism and `known_limit` until the engine can talk about them.
+- Mode: `hunk` (the function enclosing the first changed hunk on each side).
+- Provenance: `human`.
+
+Rebuild: `python benchmarks/pairs/vfc-js/build_recipes.py --crawl /tmp/secbench.jsonl` then
+`--from /tmp/secbench.jsonl`, `harvest.py --slice vfc-js --all --fetch`, `catalog_gen.py --slice vfc-js`.