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/executor.py

@@ -49,7 +49,7 @@ DUE_RESERVE = 2
 # check whether a cascade slot has freed.
 DUE_CLAIM_BLOCK_MS = 200
 #: How many cascades may be in flight, unless the service is given a number.
-#: Sustained throughput is this over the mean cascade time, so an installation
+#: Sustained throughput is this over the mean cascade time, so an instance
 #: whose nodes wait on a network rather than a CPU may want more of them —
 #: `FLOW_MAX_CASCADES` is where that is said.
 MAX_CASCADES = 4