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/sdk/cli.py

@@ -682,7 +682,7 @@ def _flow_state(flow: dict[str, Any]) -> str:
 
 
 def _portal_phrase(portal: dict[str, Any]) -> tuple[str, str]:
-    """Where this installation stands with its portal, and how to colour it.
+    """Where this instance stands with its portal, and how to colour it.
 
     Three states worth telling apart: never paired, paired and linked, and
     paired but not reaching it — the last being the one somebody needs to know
@@ -734,7 +734,7 @@ def _status_screen(client: Client) -> Any:
 
     summary = client.summary()
     flows = client.flows()
-    # Both, because an installation is usually one or the other: a live flow
+    # Both, because an instance is usually one or the other: a live flow
     # fails as an engine event, while a batch run fails on its own row.
     failures = client.events(kind="failure", limit=5)
     runs = client.runs(limit=5)