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/vibe-py/README.md

@@ -0,0 +1,23 @@
+# vibe-py slice
+
+Isolated Python web functions from [Real-Vuln-Benchmark](https://github.com/kolega-ai/Real-Vuln-Benchmark)
+(Apache-2.0 benchmark; each target repository keeps its own license).
+
+- **Vuln side**: the function enclosing a labeled vulnerable finding (`is_vulnerable = true`).
+- **Fixed twin**: a false-positive trap function (`is_vulnerable = false`) from the same repository.
+  The scanner must stay silent there, so silent-on-fix here measures precision on traps.
+- **Provenance**: `human` for the 26 human-authored apps, `agent` for the 40 LLM-generated apps.
+  The LLM-generated repositories (`kolega-ai-dev/vc-*`) currently carry **no license file**,
+  so they are skipped by `build_recipes.py` and not vendored (Req 5.4). Re-run with
+  `--allow-unlicensed` only to inspect them locally; do not commit those excerpts.
+- **Caveat**: the human corpus is educational and CTF style (DVPWA, VAmPI, vulpy, ...), not
+  production code. Say so when reading numbers.
+- **Matching**: every row names the function; the upstream scorer tolerates ±10 lines, ours
+  requires the detection inside the function.
+
+- **Excerpt shape**: line-range excerpts of methods keep their class indentation, so 27 of them
+  fail stdlib `ast.parse` ("unexpected indent") while tree-sitter parses them with correct
+  function ranges. `parse_failed 0` rests on tree-sitter; stdlib-ast parity would need dedenting.
+
+Rebuild: see the docstring of `build_recipes.py`. Regenerate the catalog with
+`python benchmarks/pairs/catalog_gen.py --slice vibe-py`. Maintainer only; CI never fetches.