feat(skill): add frontmatter fields and single SKILL.md import

[liut]

Features

• Skill gains seven frontmatter-derived columns (version, author, license, platform, category, homepage, related_skills), validated against DB widths
• skill.description widened to text (DB), 500-char soft cap at the model layer
• POST /api/skills/import: import a single SKILL.md (JSON content or multipart file, 10MiB, UTF-8); name from frontmatter, owner = current user, conflicts reject without overwrite
• import-skill CLI with --owner (username or oid, resolved against users) and --channel flags

Changes

• Frontmatter parses into (meta, body, ok); Skill.Content stores the body only, frontmatter is consumed into the seven columns
• CreateSkillWithFiles / UpdateSkillWithFiles no longer re-parse Content; callers fill the metadata columns. Package-level CreateSkill / UpdateSkill extracted for reuse
• ParseChannels reports unknown names instead of dropping them
• Skill implements comm.ModelOwner (GetOwnerID / SetOwnerID / OwnerEmpty)