Show sibling projects' versions, and let version update them

A sibling's version is its latest git tag (v1.4.0, or v1.4.0+3 for commits
after it), or, without tags, its commit's date and short hash, like
CoreDesktop's own. The version shown as available is that of the newest
commit on the default branch, which is what update installs.

sibling list shows each sibling's version. sibling check fetches every
sibling at once and shows which have a newer version, and sibling update
[name...] updates the named siblings or all of them. 71-siblings.sh now
runs sibling update, so update and version share one way of updating.

version lists the installed siblings with any newer version and, when run
in a terminal, asks for each one whether to update it now. version --check
and the update notification still only look at CoreDesktop.

The sibling skill docs describe how to release a version by tagging it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -253,7 +253,9 @@ sibling
 
 It asks for the repository's git URL, clones it into ~/.local/share/siblings/, runs the project's "install.sh" (if it has one), and confirms the install. After that, every "update" pulls the latest version of each sibling and runs its "update.sh" (if it has one).
 
-Use "sibling list" to see what's installed and "sibling remove {name}" to uninstall one.
+Use "sibling list" to see what's installed, with each project's version, and "sibling remove {name}" to uninstall one. A project's version is its latest release tag (e.g. v1.4.0), or, if it has none, the date and short hash of its latest commit, like CoreDesktop's own.
+
+"version" lists your sibling projects too, with any newer version. If one has a newer version, it asks whether to update that project now. You can also run "sibling check" to see what's newer, and "sibling update {name}" to update just that one.
 
 To make your own repository work as a sibling project, see [skills/sibling-project-format.md](skills/sibling-project-format.md).