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/api/routes/search.py

@@ -1,4 +1,4 @@
-"""One index of everything in this installation worth jumping to by name."""
+"""One index of everything in this instance worth jumping to by name."""
 
 from typing import Any, Literal
 
@@ -20,7 +20,7 @@ from fluksio.flow.secrets import get_secrets
 from fluksio.flow.store import FlowNotFound
 
 # A wall panel never reaches this route: ``deps._panel_may`` is a whitelist that
-# ends in a 403, and a whole-installation index is the opposite of what a screen
+# ends in a 403, and a whole-instance index is the opposite of what a screen
 # on a wall is allowed to read.
 router = APIRouter(
     prefix="/search", tags=["search"], dependencies=[Depends(get_current_user)]
@@ -144,7 +144,7 @@ async def read_search_index(
     """Everything searchable, for the client to match against as it is typed.
 
     The whole index rather than a query: it is a few hundred short rows for an
-    installation of any ordinary size, so one fetch when the panel opens beats a
+    instance of any ordinary size, so one fetch when the panel opens beats a
     round trip per keystroke — and the client already has a matcher.
 
     Secrets are named only to a superuser, which is who ``/secrets`` answers to.