fix(harness): preserve RuntimeContext during skill promotion (#2465)

### Background

Skill promotion did not consistently preserve the caller's
`RuntimeContext`.

For USER- or SESSION-scoped workspaces, the context determines which
filesystem namespace contains the draft skill. Although
`SkillPromoter.promote(...)` accepted a context, the following
operations still used the default context or `RuntimeContext.empty()`:

- Draft skill lookup
- Supporting resource discovery and reading
- Moving the approved skill into the live directory

This could cause a user-scoped draft to appear missing or make promotion
operate on the wrong namespace.