Add channels metadata: membership and build-channel rules for edge/rc/stable

A package's .omarchy/package.json may now pin where it lives with
"channels": [...]. With the key present the package builds in each listed
channel except stable (stable is only fed by promotion); without it, today's
defaults hold (build for edge; fast-ring also builds stable directly).

package_moves_to_channel() is the advance/promote eligibility rule: a member
of the destination channel that is not built there natively.

The release pair (omarchy, omarchy-settings) is edge+rc+stable — edge stays
during the client-migration overlap window and drops later. The dev pair is
pinned to edge only.

pkgbuilds/omarchy-settings-dev/.omarchy/package.json

@@ -1,3 +1 @@
-{
-  "source": "local"
-}
+{ "source": "local", "channels": ["edge"] }