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/code/api.md

@@ -5,7 +5,7 @@ client of this schema, not a privileged path into the engine — so anything you
 can click, you can script.
 
 Base URL: `https://api.${DOMAIN}/api/v1`, or `http://127.0.0.1:8000/api/v1` for
-a `fluksio serve` installation.
+a `fluksio serve` instance.
 
 ```sh
 export FLUKSIO=http://127.0.0.1:8000/api/v1
@@ -34,7 +34,7 @@ Agents authenticate differently — see [Agents over MCP](agents.md).
     the tour.
 
     It is closed in production on purpose: the schema enumerates every endpoint
-    the installation serves, including the paths webhook nodes mounted at
+    the instance serves, including the paths webhook nodes mounted at
     runtime.
 
 ## Flows
@@ -230,7 +230,7 @@ draw from.
 | `404` | no such flow, dashboard, run or message |
 | `409` | someone else saved first — the body carries `current_version` |
 | `422` | a parameter, port or binding did not typecheck |
-| `503` | that subsystem is not available on this installation |
+| `503` | that subsystem is not available on this instance |
 
 A 409 on a save or a publish is not an error to retry blindly: it means the
 stored version moved past the one you were editing. Re-read, merge, save again.
@@ -239,4 +239,4 @@ stored version moved past the one you were editing. Re-read, merge, save again.
 
 The frontend's TypeScript client is generated from the OpenAPI schema
 (`make generate-client`). Any OpenAPI generator will do the same for your
-language — point it at `/api/v1/openapi.json` on a non-production installation.
+language — point it at `/api/v1/openapi.json` on a non-production instance.