SmolDataEnvs: fill empty file lists in 424 rows

`files: []` appears in 424 rows: 398/5,000 train, 15/144 eval, and 11/250 test. Every row has data, but the GitHub single-program rollout builds its prompt's file list from `files`, so these prompts listed no file names.

| Rows | Before | This PR |
|---:|---|---|
| 213 | `instruction` has a placeholder; no file name appears anywhere in the row | Put every staged file's flattened basename in `files` and the instruction's list |
| 211 | `instruction` names the files, sometimes with spaces or brackets, but `files` is empty | Put those named, staged files in `files`; they can be a subset of all staged files |

Only these rows change, in `files` and the 213 placeholder instructions. Schema, metadata, and parquet layout stay the same. No empty `files` list or placeholder remains.

In a check with base Qwen3-32B that listed every staged file, 38 affected tasks (15 eval, 23 from the GRPO training set) went from 0/304 to 79/304 correct. The companion GitHub PR has the details: https://github.com/adithya-s-k/FineEnvs/pull/21.

### Testing

The file names from the 211 instruction-list rows were checked against staged files. On test, 11 rows change for the single-program prompt; `instruction` changes only for the 5 placeholder rows. Single-program test scores can move.

data/eval-00000-of-00001.parquet

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+size 48469

data/test-00000-of-00001.parquet

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data/train-00000-of-00001.parquet

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