examples/file_io.vera litters the repo root with hello.txt on every run

Test/example hygiene, surfaced twice during the #1213 burndown (two agents caught `hello.txt` in `git add -A` staging and had to unstage manually). `examples/file_io.vera` writes the relative path `hello.txt`, which `vera run` resolves against the process CWD — so any run from the repo root (`pytest tests/`, `python scripts/check_examples_readme.py`, a bare `vera run examples/file_io.vera`) leaves an untracked file in the repository root. Fix (smallest change that kills the actual reported pain): add `/hello.txt` to `.gitignore` — ignored files are invisible to `git add -A` and `git status --short`, which is precisely what bit. The fixing pass should also (a) check whether any OTHER example writes relative paths (sweep `IO.write_file`/`write` calls across examples/), and (b) verify a full `pytest tests/ -q` from the repo root leaves `git status --short` clean afterwards. If the sweep finds more litter sources, upgrading to a chdir-into-tempdir harness for example-running scripts is the fuller fix — the fixing agent decides with the evidence. No compiler behaviour changes; `Skip-changelog:` applies if the diff is gitignore-only. Lands in the burndown's hygiene PR (C.6, alongside #1228).