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/pipeline.py

@@ -210,7 +210,7 @@ def _derive(
 
     The two lookups come out of the same walk: who consumes a message, and
     which node an id names. Both were linear scans over every node in the
-    installation, on the per-message path.
+    instance, on the per-message path.
     """
     # A message may have several producers; every one of them is upstream
     # of the nodes consuming it.