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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015YrQnKV3bnQd4K342y8tKj

frontend/src/components/Common/OverviewToolbar.tsx

@@ -303,10 +303,10 @@ export function ConfirmDelete({
           <DialogDescription>
             {description ?? (
               <>
-                {names.length === 1 ? "It goes" : "They go"} from the
-                installation at once. The store's git history keeps what was
-                there, but nothing in the app brings{" "}
-                {names.length === 1 ? "it" : "them"} back.
+                {names.length === 1 ? "It goes" : "They go"} from the instance
+                at once. The store's git history keeps what was there, but
+                nothing in the app brings {names.length === 1 ? "it" : "them"}{" "}
+                back.
               </>
             )}
           </DialogDescription>