fix(skill_engine): keep SKILL.md when *** File: appears in prose

[santhreal]

Problem

A skill body that mentions a single mid-document *** File: line (for example inside an example) was parsed as multi-file FULL output. create_skill then wrote only that file and never created SKILL.md.

Repro

from pathlib import Path
from openspace.skill_engine.patch import create_skill

content = "# My Skill\n\n*** File: helper.sh\n#!/bin/bash\necho hi\n"
create_skill(Path("/tmp/s"), content)
# helper.sh exists; SKILL.md does not

Fix

Align detect_patch_type with its docstring (≥2 markers, or one at content start with a body). For a single mid-document *** File:, parse_multi_file_full keeps the whole content as SKILL.md.