Retire the elephant packages and omarchy-walker (#872)

Omarchy 4 no longer uses Elephant or the omarchy-walker meta package, and
omarchy-upgrade-to-quattro removes them; every channel serves Omarchy 4.0.4.
Nothing else here depends on them. walker itself stays.

Deleting a recipe stops it building but leaves it in the channel databases,
and the only removal tool worked on the old repository host's local tree.
Add bin/unpublish-packages, publish-artifact's counterpart: pull the channel
database, repo-remove every entry built from the named pkgbases, upload it.
Package files stay in the bucket. unpublish.yml runs it per channel and
architecture under the publish lock, for packages with no recipe on master.

README.md

@@ -565,12 +565,26 @@ bin/repo deploy                      # Build locally, then publish from the host
 bin/repo push                        # Upload local builds to the host and publish
 bin/add-package <package>            # Add an Omarchy-owned package with metadata
 bin/package-worktree <package>       # Inspect historical AUR provenance in a scratch workspace
-bin/repo remove <package>            # Remove package
+bin/repo remove <package>            # Remove package (host workflow; CI uses unpublish.yml)
 bin/sync-upstream                    # Update packages that track a vendor release feed
 bin/sync-rebuilds                    # Bump pkgrel for packages whose dependencies moved
 bin/clean-docker                     # Clear Docker images/cache (forces fresh rebuild)
 ```
 
+### Retiring a package
+
+Delete its recipe directory in a PR. Once that merges, take it out of the
+channel databases with the **Unpublish retired packages** workflow:
+
+```
+gh workflow run unpublish.yml -f packages="<pkgbase> ..." -f channels="edge rc stable"
+```
+
+It removes every entry built from those pkgbases (split outputs and `-debug`
+included) from both architectures' databases, and refuses any package that
+still has a recipe on master. Package files stay in the bucket: published
+filenames are immutable, and nothing references them once the entries are gone.
+
 ### Package Metadata Tools
 
 ```bash