fix: show the container hostname in the shell prompt

Coolify injects HOST=0.0.0.0 into every compose app, and zsh seeds $HOST and
the %m/%M prompt escapes from that variable rather than calling gethostname().
The prompt read "0", the first dot-separated field of 0.0.0.0, even though the
container hostname itself was set correctly.

Pass the hostname in as HOST alongside the hostname: key, both from a single
SERVICE_HOSTNAME variable. Neither service reads HOST itself -- code-server
binds [::]:8443, Paseo binds PASEO_LISTEN -- so this only affects the prompt.

Not named HOSTNAME: Compose interpolation lets the deploying shell's
environment win over the .env file, and HOSTNAME is set in every container,
including the one Coolify runs in.

PASEO_LABEL is renamed to SERVICE_HOSTNAME; it was never a Paseo variable.
The example git identity is blanked out along with it.

paseo/compose.yml

@@ -1,9 +1,11 @@
 services:
   paseo:
     build: .
-    hostname: ${PASEO_LABEL}
+    hostname: ${SERVICE_HOSTNAME}
     environment:
       - TZ=${TZ}
+      # Hostname zsh's prompt reads, overriding the one Coolify injects.
+      - HOST=${SERVICE_HOSTNAME}
       - SHELL=${SHELL}
       - PASEO_PASSWORD=${PASEO_PASSWORD}
       - PASEO_HOSTNAMES=${PASEO_HOSTNAMES}