hosts: decommission ca host and remove labmon

Remove the step-ca host and labmon flake input now that ACME has been
migrated to OpenBao PKI.

Removed:
- hosts/ca/ - step-ca host configuration
- services/ca/ - step-ca service module
- labmon flake input and module (no longer used)

Updated:
- flake.nix - removed ca host and labmon references
- flake.lock - removed labmon input
- rebuild-all.sh - removed ca from host list
- CLAUDE.md - updated documentation

Note: secrets/ca/ should be manually removed by the user.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

rebuild-all.sh

@@ -5,7 +5,6 @@ set -euo pipefail
 HOSTS=(
     "ns1"
     "ns2"
-    "ca"
     "ha1"
     "http-proxy"
     "jelly01"