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

backend/fluksio/flow/nodes/base.py

@@ -179,7 +179,7 @@ class Node:
     idempotent: bool = True
 
     @classmethod
-    def instance_key(cls, params: dict[str, Any]) -> str | None:
+    def target_key(cls, params: dict[str, Any]) -> str | None:
         """Which outside thing these parameters point at, if any.
 
         Two nodes with the same key talk to the same broker topic, URL or