Fix RemoteSupport systemd state directory bootstrap

[KarotsSugarpie]

Fixes RemoteSupport failing with systemd status 226/NAMESPACE on freshly updated cabinets.

Root cause:

• the unit declared ReadWritePaths=/var/lib/pincabos-remote-support ...
• that directory does not exist on a fresh cabinet
• systemd mount namespace setup fails before the agent can create it

Fix:

• use StateDirectory=pincabos-remote-support
• use StateDirectoryMode=0700
• remove the fragile ReadWritePaths dependency
• retain ConditionPathExists=/var/lib/pincabos-link/device.json
• extend RemoteSupport CI to require StateDirectory and prevent reintroducing the broken ReadWritePaths line

Observed failure on Alpha 4.82:
Failed to set up mount namespacing: /var/lib/pincabos-remote-support: No such file or directory
status=226/NAMESPACE