V3 Python Black format-check capability pack. By-value connector composition (service_kind=cli, no separate ingredient): one local black CLI ingredient exposes bounded project operations for readiness and format-check diagnostics. Format checks inspect one trusted Python file or directory from one explicit project directory cwd and return Black text/diff diagnostics via the python.black_format_check catalog operation. Fixed-function and non-writing: check mode, diff output, no color, no cache reads or writes, no stdin, code string, line-range selector, target-version override, config path override, include/exclude override, worker override, preview/unstable style, required-version override, arbitrary flags, environment override, or shell wrapper. Would-reformat results are represented as successful text diagnostics; internal Black errors still fail. Requires black on PATH.