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/ConnectionNotice.tsx

@@ -9,7 +9,7 @@ import { isPortal } from "@/lib/portal"
 const ID = "connection-offline"
 
 /**
- * Says when the installation cannot be reached, and when it was last heard
+ * Says when the instance cannot be reached, and when it was last heard
  * from.
  *
  * Only ever meaningful under a portal: a local install cannot lose contact with
@@ -39,8 +39,8 @@ export function ConnectionNotice() {
     }
     notify(
       connection.lastSeen
-        ? `Installation offline — last seen ${ago(connection.lastSeen / 1000)}, reconnecting…`
-        : "Installation offline — reconnecting…",
+        ? `Instance offline — last seen ${ago(connection.lastSeen / 1000)}, reconnecting…`
+        : "Instance offline — reconnecting…",
       "warning",
       { id: ID, persistent: true },
     )