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/runs.py

@@ -296,7 +296,7 @@ def _from_run(
             if artifacts is not None and artifacts.path(row.digest) is None:
                 raise RunRejected(
                     f"Parameter '{key}': run '{run_id}' made '{output}', but its "
-                    "bytes are gone from this installation's store"
+                    "bytes are gone from this instance's store"
                 )
             return {
                 "digest": row.digest,
@@ -327,7 +327,7 @@ def _from_digest(
     if artifacts is not None and artifacts.path(digest) is None:
         raise RunRejected(
             f"Parameter '{key}': '{digest}' is known but its bytes are gone "
-            "from this installation's store"
+            "from this instance's store"
         )
     return {
         "digest": row.digest,