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/tests/flow/test_brain_graph.py

@@ -78,7 +78,7 @@ def test_an_edge_carries_the_qualified_message(controller: FlowController):
 def test_a_credential_never_reaches_the_key():
     # Stored params, so a secret is still a reference. Neither its name nor its
     # value belongs in something the browser gets to see.
-    key = MqttNode.instance_key({**BROKER, "password": {"$secret": "broker_pw"}})
+    key = MqttNode.target_key({**BROKER, "password": {"$secret": "broker_pw"}})
 
     assert key == "mosquitto:1883/sensors/temp"