feat(multi-site): spoke.env.example + CFG_PUBLIC_DOMAIN

A dedicated spoke.env for the join-a-cluster vars (CFG_MASTER_DIRECTORY_URL/
_JOIN_KEY, CFG_SPOKE_NO_INBOUND/_PUBLIC_HOST, CFG_PUBLIC_DOMAIN), split out
of setup.env purely for clarity -- setup.env still has every option and
keeps working as a single file if that's preferred. setup.sh reads both
(setup.env first, spoke.env layered on top so its values win), same
first-run-only rule as setup.env already had.

Also adds CFG_PUBLIC_DOMAIN (documented in MULTI_SITE_SPEC.md §4 but never
actually wired into setup.sh): an inbound spoke/standalone site's own public
web domain, independent of CFG_DOMAIN (the shared LDAP identity namespace,
which must stay identical across every site). Unset behaves exactly as
before -- hostnames derive from CFG_DOMAIN like any standalone install.

.gitignore

@@ -19,6 +19,11 @@ proxy.env
 # per-deployment and is not committed.
 setup.env
 
+# spoke.env — same rule as setup.env, but for the join-a-cluster vars split
+# out for clarity (spoke.env.example IS committed). Holds a real site join
+# key once filled in.
+spoke.env
+
 # Backup artifacts (hold secrets — the whole user directory + Redis dumps)
 *.rdb
 *.ldif