Stage now writes through os.OpenRoot(workspace) instead of a resolve-then-write
check. SKILL.md and schema.json are capped at MaxFileSize. listFiles and
copySiblings share one walk so Files and the staged tree agree on nested skills
and .git. Also: Option renamed WalkOption, DefaultSkipDirs is a function,
ValidateGlob caps ** segments, Render drops spec keys from Extra, SourceHash
delimits its inputs, and standalone .md no longer picks up a sibling schema.