Save CLI results with their own file extension, not the input's #587

Why

When an Assembly turns one input into a result with a different type, the CLI saves the result under the input's filename and extension. A /video/thumbs Step on social.mp4 with -o out/ wrote a JPEG to out/…/social.mp4; a builtin/transcribe run saved its JSON as ….mp4. Opening those files fails or confuses tools, and users can't tell what they got.

Seen with @transloadit/node 6.1.2 on 2026-10-18.

Proposal

• Name downloaded results by the result's own name/ext (or the input basename plus the result's extension), not the input's extension.
• Keep the existing directory layout (<input>/<step>/…) so scripts don't break, and add a test with a type-changing Step (video → jpg, video → json).

Implementation is in #611. Red-first CLI behavior tests cover video results saved as JPEG and JSON with their own names/extensions, while preserving existing parent and input/Step directories. A live /video/thumbs CLI run also saved testsrc_0.jpg with verified JPEG bytes and no .mp4 output. Final CI passes all 14 checks on 1d5bb2aa; local checks pass. The PR closes this issue when merged.

_{🤖 Claimed by kvz on studio1/node-sdk since 2026-10-23 · resume: codex resume 01a16a73-d73f-7e11-83fe-db48d62fa526}