feat(hosts): add os_hostname field to all host.yaml files

Declares the real OS-level hostname for each node so deploy-node.sh
can resolve the correct hosts/ directory without assuming
OS-hostname == logical name. Only VPS differs: its OS hostname is
ubuntu-4gb-hel1-1 while the repo directory is hosts/vps/.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/chelsty-ha/host.yaml

@@ -1,4 +1,5 @@
 hostname: chelsty-ha
+os_hostname: chelsty-ha
 site: chelsty
 
 roles:

hosts/chelsty-infra/host.yaml

@@ -1,4 +1,5 @@
 hostname: chelsty-infra
+os_hostname: chelsty-infra
 site: chelsty
 
 roles:

hosts/piha/host.yaml

@@ -1,4 +1,5 @@
 hostname: piha
+os_hostname: piha
 
 roles:
   - infra

hosts/saturn/host.yaml

@@ -1,4 +1,5 @@
 hostname: saturn
+os_hostname: saturn
 
 roles:
   - control

hosts/solaria/host.yaml

@@ -1,4 +1,5 @@
 hostname: solaria
+os_hostname: solaria
 
 roles:
   - compute

hosts/vps/host.yaml

@@ -1,4 +1,5 @@
 hostname: vps
+os_hostname: ubuntu-4gb-hel1-1
 
 roles:
   - edge