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/controller.py

@@ -98,7 +98,7 @@ HOOK_PREFIX = "/hooks"
 NODE_STOP_TIMEOUT = 5.0
 
 # How long a rebuild asked for by a request waits for one already running.
-# Generous on purpose: a rebuild of a populated installation reconnects every
+# Generous on purpose: a rebuild of a populated instance reconnects every
 # node and takes the better part of ten seconds, and a caller queued behind a
 # healthy one of those should not be turned away. Past that the controller is
 # wedged rather than busy, and an error the caller can act on beats a request
@@ -106,7 +106,7 @@ NODE_STOP_TIMEOUT = 5.0
 REBUILD_WAIT = 15.0
 
 # How long a caller waits for a *flow* rebuild that is already running. A
-# per-flow rebuild reconnects one flow's nodes rather than the installation's,
+# per-flow rebuild reconnects one flow's nodes rather than the instance's,
 # so this is a queueing budget — several of them back to back, which is what
 # seeding does — not the room a single one needs. Fifteen seconds was sized for
 # the whole-pipeline rebuild and would let a wedge sit unreported.
@@ -680,7 +680,7 @@ class FlowController:
         another flow owns — but the wiring is derived from message names, so
         swapping one flow's nodes into the graph and deriving the edges again
         is enough. What that saves is the reconnecting: the cost of a rebuild
-        on a populated installation is every node opening its socket again,
+        on a populated instance is every node opening its socket again,
         and only one flow's have changed.
 
         A flow the store no longer has is taken out instead of replaced.
@@ -1522,12 +1522,12 @@ class FlowController:
                 key = None
                 if node_type is not None:
                     try:
-                        key = node_type.cls.instance_key(node_def.params)
+                        key = node_type.cls.target_key(node_def.params)
                     except Exception:
                         # A plugin's own grouping is not worth the whole graph;
                         # this node just stands on its own.
                         logger.exception(
-                            "instance_key failed for node type '%s'", node_def.type
+                            "target_key failed for node type '%s'", node_def.type
                         )
                 gid = f"{node_def.type}:{key}" if key else member
                 gid_of[member] = gid