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

docs/index.md

@@ -52,6 +52,6 @@ and a training pipeline on it.
 | Drive it from Python, a shell or CI | [Code and the CLI](code/cli.md) |
 | Look up a node type or a payload type | [Reference](reference/node-types.md) |
 
-Fluksio is self-hosted by default. An installation runs offline, keeps its data
+Fluksio is self-hosted by default. An instance runs offline, keeps its data
 on its own disk, and never contacts anything unless you
 [connect it to a portal](interface/portal.md) yourself.