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/GlobalSearch.tsx

@@ -58,7 +58,7 @@ function hint(entry: SearchEntry): string {
 }
 
 /**
- * Everything in this installation, by name, from anywhere.
+ * Everything in this instance, by name, from anywhere.
  *
  * The whole index arrives in one fetch and `cmdk` does the matching, so results
  * narrow as they are typed without a round trip per keystroke.
@@ -190,7 +190,7 @@ export function GlobalSearch({
           </>
         ) : (
           <p className="py-6 text-center text-sm text-muted-foreground">
-            Start typing to search this installation.
+            Start typing to search this instance.
           </p>
         )}
       </CommandList>