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/Health/HealthOverview.tsx

@@ -75,7 +75,7 @@ export function HealthOverview({
   const queue = (summary?.queue ?? {}) as Record<string, number>
   const degraded = summary?.status === "degraded"
   const errors = (flows ?? []).reduce((total, row) => total + row.errors, 0)
-  // An installation nobody has run a batch flow on has no runs tile at all —
+  // An instance nobody has run a batch flow on has no runs tile at all —
   // the same "nothing has been run yet" the Runs screen itself goes by.
   const running = (runs ?? []).reduce((total, row) => total + row.running, 0)
   const queued = (runs ?? []).reduce((total, row) => total + row.queued, 0)