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.

setup.env.example

@@ -61,9 +61,19 @@ CFG_DOMAIN=example.com
 # 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.
+#
+# All of these (and the no-inbound relay pair below) also live in their own
+# spoke.env.example, if you'd rather keep join-a-cluster config in a
+# dedicated file instead of here -- both are read, spoke.env's values win.
 #CFG_MASTER_DIRECTORY_URL=https://sso.master.example.com
 #CFG_MASTER_DIRECTORY_JOIN_KEY=stj_9f2e...
 
+# This site's own public web domain, independent of CFG_DOMAIN above (the
+# shared LDAP identity namespace, which must be identical across every site).
+# Optional -- only meaningful for an inbound spoke/standalone site that wants
+# its own domain rather than sharing the master's.
+#CFG_PUBLIC_DOMAIN=branch2.example.com
+
 # No public IP at all (CGNAT, etc.)? The master can still reach this spoke by
 # relaying over the gateway-to-gateway WireGuard mesh instead of the open
 # internet (MULTI_SITE_SPEC.md §5.2) -- but the mesh peering itself is a