Import a skill from a local .skill/.zip archive: POST /api/skills/import now accepts a multipart upload (file + on_conflict) alongside the JSON URL body, and the CLI gains `multica skill import --file <path>`. Reuses the existing create + on_conflict contract, per-file/bundle/count caps, reserved-SKILL.md rule, and a zip-slip guard.
Closes#4730
MUL-3865