Return every file in the project's output directory as a flat list.

This backs the All-data browser: a fallback view of the run's real directory structure, so a file is reachable even when no gallery classifies it. Local disk and S3 are merged — a run that has been evicted from local cache still lists from S3, and an in-flight run lists files that have not been uploaded yet.

Dot-directories (.venv, .git, .claude, …) and their non-dot equivalents are always skipped — they are machine noise, never run output.

Paths are relative to the project's output/ root, matching what GET /api/projects/{id}/files/{path} expects.