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/components/Flow/queries.ts

@@ -100,7 +100,7 @@ export function useParamSuggestions(
     staleTime: SUGGEST_STALE,
   })
   // ponytail: reads every flow to collect them, sharing the editor's own cache
-  // entries; an aggregate endpoint if a big installation makes that hurt.
+  // entries; an aggregate endpoint if a big instance makes that hurt.
   const details = useQueries({
     queries: (type ? (flows?.data ?? []) : []).map((flow) => ({
       ...flowQueryOptions(flow.name),