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/getting-started/data-science.md

@@ -14,7 +14,7 @@ pip install fluksio
 fluksio serve
 ```
 
-That is the whole installation. No Docker, no database server, no ports to
+That is the whole instance. No Docker, no database server, no ports to
 open, and no login. The first run prints something like:
 
 ```text
@@ -25,7 +25,7 @@ Fluksio 0.1.0 — data in /home/you/my-research/.fluksio
   API      http://127.0.0.1:8000/api/v1
   Nodes    /home/you/my-research/.venv/bin/python
            your environment, adopted. Add packages with pip.
-  No portal. Pair this installation with:
+  No portal. Pair this instance with:
     fluksio enroll <code>
   Signed in as admin@example.com
            token in /home/you/my-research/.fluksio/client.json
@@ -39,7 +39,7 @@ below just works. `fluksio login` is for an engine somewhere *else*.
 **Write that password down** anyway. It is shown once, and it is what the
 dashboard asks for.
 
-### One installation per project
+### One instance per project
 
 `.fluksio` sits beside your code, and is found the way `.git` is — from the
 directory you are standing in, or any directory above it. So two repositories
@@ -135,7 +135,7 @@ environment it did not make.
 
 ### A venv of Fluksio's own
 
-Sometimes you want the isolation instead: a shared installation, a container,
+Sometimes you want the isolation instead: a shared instance, a container,
 or an environment too precious to let a node's dependency near. Set
 `NODE_VENV=managed` and Fluksio builds and owns one under the data directory:
 
@@ -761,14 +761,14 @@ A submit is around 15 ms, so calling that in a loop is a reasonable thing to do.
 
 The pip install gives you the engine and the API, not a web interface — a
 machine with no inbound route cannot serve one usefully anyway. To see the
-canvas, the run history and live loss curves, pair the installation with a
+canvas, the run history and live loss curves, pair the instance with a
 portal, which serves the dashboard from its side:
 
 ```sh
 fluksio enroll <claim-code> --portal https://hub.fluksio.com
 ```
 
-Get the claim code from the portal under **Installations → Add installation**.
+Get the claim code from the portal under **Instances → Add instance**.
 Nothing needs to be exposed: your machine dials out and holds the connection
 open. See [Accounts and the portal](../interface/portal.md).