thumbnail-skill

# thumbnail-skill Machine interface: `thumbnail run - --json` with a `{"tool": "thumbnail/<name>", "params": {...}}` document on stdin; exactly one response document on stdout. `thumbnail skill --json` prints the contract, `thumbnail doctor --json` the environment (fonts, ffmpeg-skill, path policy). Tools: `thumbnail/validate` (structural check only), `thumbnail/render` (canvas + assets + elements -> PNG/JPEG), `thumbnail/extract_frame` (one video timestamp -> one frame, nothing else). Rules for a calling agent: 1. Decide everything about content before calling this skill: which still image, or which exact video timestamp (`asset.timestamp`, seconds); what the text says (with literal `\n` for line breaks — this skill never wraps or reflows text); where every element sits (`position`, `size`, `z_index`). This skill renders exactly what it is given and searches for nothing. 2. Reference fonts by `font_id` from the registry (`thumbnail skill --json` -> `fonts.font_ids`, or `thumbnail doctor --json` for what actually resolves on this machine). A request never carries a font path; an unresolvable `font_id` fails `MISSING_INPUT`, never a silent substitution. 3. Never send `command`, `argv`, `shell`, `filter`/`filter_complex`/`vf`/`af`, `executable`, `env`, or HTML/CSS/JS: the request is rejected with `INVALID_REQUEST` wherever such a key appears. 4. Keep outputs inside the workspace (`--workspace` / `options.workspace`), never at an input path; set `overwrite: true` deliberately. Restrict inputs with `options.allowed_input_roots` when the caller's own inputs come from an untrusted or shared location. 5. Read `output`, `sha256`, `width`/`height`, `reused` and `provenance` from the response; the exit code is 0 only when `ok` is true. A `video_frame` asset needs ffmpeg-skill reachable (env var `THUMBNAIL_SKILL_FFMPEG_SKILL_DIR` or `--ffmpeg-skill`); a still-image-only document never needs it. See README.md for the ThumbnailDocument schema, element/font/output tables, error codes, security boundary and the responsibility split with video-production-agent and the other media skills.