feat(multi-site): wire selfUrl through setup.sh so joins register live

Extends the shipped CFG_MASTER_DIRECTORY_URL/JOIN_KEY join flow with the
selfUrl a spoke needs to register itself for live catalog replication
(theta-directory v2.4.0's POST /api/site/spokes) -- without this, every
spoke was permanently limited to the one-time join snapshot even after
the master gained the ability to push live updates.

setup.sh already computes CFG_SSO_HOST before this point in the script;
passes https://$CFG_SSO_HOST as bootstrap/site-join.js's third argument,
which forwards it as `selfUrl` in the POST /api/site/join body.

setup.env.example

@@ -55,7 +55,12 @@ CFG_DOMAIN=example.com
 # -> Mint key). Honored ONLY on a first-run bring-up (before ./config/ exists),
 # so it can never merge an already-populated directory; re-runs ignore it.
 # The spoke adopts the master's users/groups/resources and persists its spoke
-# role in ./config/site.json (isMaster=false, masterUrl, siteSlug).
+# role in ./config/site.json (isMaster=false, masterUrl, siteSlug). It also
+# registers itself with the master (using this site's own CFG_SSO_HOST) so
+# future catalog changes on the master get pushed here live instead of this
+# being a one-time snapshot -- the master must be able to reach THIS site's
+# CFG_SSO_HOST for that part to work; if it can't (this site has no inbound
+# path), the join still succeeds, it just never receives live updates.
 #CFG_MASTER_DIRECTORY_URL=https://sso.master.example.com
 #CFG_MASTER_DIRECTORY_JOIN_KEY=stj_9f2e...