Use `_tasks.tsv` to assign task labels

And filenames should use `task-01`, `task-02`, etc. And allow values in `_tasks.tsv` to specify dataset-wide classification

(Would need to `.bidsignore` this file)

Start by locating the existing task-label and filename handling, then check how `_tasks.tsv` is discovered and parsed. Verify the expected `task-01`, `task-02` naming and dataset-wide classification values, and ensure `_tasks.tsv` is excluded through `.bidsignore`; done means the reorganized BIDS layout reflects those values without treating the metadata file as input data.

Written by the indexing model from the issue text.