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.

code-server/.env.example

@@ -2,6 +2,10 @@
 #
 #   cp .env.example .env
 
+# Container hostname, also passed in as HOST -- the name zsh's prompt shows.
+# Not named HOSTNAME: the deploying shell's own HOSTNAME would override it.
+SERVICE_HOSTNAME=code-server
+
 # Web UI login password. MUST NOT be blank -- the LinuxServer image serves
 # code-server without authentication when PASSWORD is empty.
 # Also used for SUDO_PASSWORD inside the container.