Support Black in legacy linter routing #576

detect already recognizes Black (src/detect/scan.ts), but the legacy-linter-recipes change refuses it: taskless agent create-legacy-rule --tool=black exits with a pointer elsewhere, because Black is a formatter and cannot hold a rule.

That leaves a gap for Python repositories that run Black. A formatting-shaped request ("use double quotes", "wrap at 100 columns", "trailing commas in multi-line calls") is often already settled by Black or by one of its few options (line-length, target-version, skip-string-normalization, skip-magic-trailing-comma, preview). Today an agent may write an sg rule that fights the formatter, or one that duplicates what it already does.

Support would mean:

• A create-legacy-rule.black.md recipe whose job is step 1 of the legacy ladder: decide whether Black already settles the request, or would with one of its options, and say so instead of writing a rule.
• A route paragraph for Black when detect finds it.
• Removing black from the refused --tool names.

The open question is the hand-off when Black does not settle the request: send it back to route, or treat formatting requests as out of scope for Taskless rules entirely.

Follows the legacy-linter-recipes OpenSpec change (Ruff, ESLint, Biome, pylint, flake8 at launch).