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/Flow/useFlowSocket.ts

@@ -32,7 +32,7 @@ type FlowEvent =
       nodes: { id: string; status: string; error?: string | null }[]
       paused?: string[]
       logs?: LogLine[]
-      /** Emission counts per qualified node. Absent on older installations. */
+      /** Emission counts per qualified node. Absent on older instances. */
       emits?: Record<string, number>
     }
   | {
@@ -194,7 +194,7 @@ function connect() {
         liveStore.setStatuses(message.nodes ?? [])
         liveStore.setPausedFlows(message.paused ?? [])
         liveStore.setLogs(message.logs ?? [])
-        // Missing against an installation older than this bundle; the graph
+        // Missing against an instance older than this bundle; the graph
         // then starts from zero the way it always did.
         liveStore.setEmits(message.emits ?? {})
         break
@@ -321,7 +321,7 @@ function connect() {
       const payload = JSON.parse(event.data)
       if (payload?.type === "batch") {
         // A cascade publishes a dozen events at once and the engine coalesces
-        // them into one frame. An installation older than this bundle sends
+        // them into one frame. An instance older than this bundle sends
         // them one at a time, which is the branch below.
         for (const message of payload.events ?? []) handle(message)
       } else {
@@ -347,7 +347,7 @@ function connect() {
         return
       }
     }
-    // 1013 is the portal saying the installation is not attached — the one
+    // 1013 is the portal saying the instance is not attached — the one
     // close code that means "offline" rather than "the socket dropped".
     if (event.code === 1013) {
       connectionStore.setOffline(null)