core+cli: castle apply — one converge verb replaces deploy/start/enable/…

Reconcile the running system to declared config in a single operation,
replacing the old deploy && start plus the per-kind enable/disable/
install/uninstall verbs. The mechanism varies by manager (systemd unit /
PATH install / gateway route); the verb never does.

Core (castle_core.deploy.apply):
- render (units, Caddyfile, tunnel) then reconcile: activate every
  enabled deployment that's down, restart any whose unit bytes changed,
  deactivate the disabled ones. Returns ApplyResult (the enacted diff).
- --plan computes the diff and touches nothing.
- restart-on-change is exact: _render_unit_files is the single source of
  truth for unit bytes, used both to write units and to predict restarts,
  so the plan can't drift from what deploy writes.

Desired state:
- DeploymentBase.enabled (default True) — the declarative on/off. apply
  converges to it; a disabled deployment is defined but not running and
  gets no gateway route (else it would 502). Registry omits enabled when
  True, keeping existing registries byte-identical.

CLI:
- add `castle apply [name] [--plan]`; keep `restart` as the one
  imperative bounce, plus status/doctor/logs/gateway/program.
- retire deploy, start, stop (top-level); service/job deploy|enable|
  disable|start|stop; tool install|uninstall; program install|uninstall.
- next-step strings + doctor hints now say `castle apply`.

Verified: core 129 + cli 31 green; live `castle apply` on a converged
node reports "nothing to do" with no spurious restarts; --plan shows an
empty diff; gateway + api stay up.

cli/src/castle_cli/commands/run_cmd.py

@@ -82,12 +82,12 @@ def run_run(args: argparse.Namespace) -> int:
 
     # Service: deployed command from the registry.
     if not REGISTRY_PATH.exists():
-        print("Error: no registry found. Run 'castle deploy' first.")
+        print("Error: no registry found. Run 'castle apply' first.")
         return 1
 
     registry = load_registry()
     if name not in registry.deployed:
-        print(f"Error: '{name}' is not a deployed service. Run 'castle deploy' first.")
+        print(f"Error: '{name}' is not a deployed service. Run 'castle apply' first.")
         return 1
 
     deployed = registry.deployed[name]