Rename Installation to Instance

Follows the portal: the noun is "instance" everywhere the app says it —
UI strings, CLI output, error details, docs and comments. The wire keys
(`instance_id`, `instance_token`) and the hub route this calls move with it.

An existing cloud.json is adopted rather than refused: without the key
alias the dataclass fails to parse, which the caller swallows and reads as
"never enrolled" instead of "reconnect".

`instance_key` on a node type becomes `target_key`. It means the outside
thing a node points at, which is a different sense of the word, and keeping
both would put two meanings of "instance" in one codebase.

frontend/src/utils.ts

@@ -4,13 +4,13 @@ import { ago } from "./components/Health/queries"
 import { offlineDetail } from "./lib/connectionStore"
 
 function extractErrorMessage(err: ApiError): string {
-  // An unreachable installation is not a rejected action: say plainly that
+  // An unreachable instance is not a rejected action: say plainly that
   // nothing was delivered, so nobody is left wondering whether it half-landed.
   const offline = offlineDetail(err)
   if (offline) {
     return offline.lastSeen
-      ? `Not delivered — the installation is offline (last seen ${ago(offline.lastSeen)})`
-      : "Not delivered — the installation is offline"
+      ? `Not delivered — the instance is offline (last seen ${ago(offline.lastSeen)})`
+      : "Not delivered — the instance is offline"
   }
 
   if (err instanceof AxiosError) {