feat(agentnetwork): store the endpoint as domain + proxy address

Replace the settings row's (cluster, subdomain) identity columns with
(domain, proxy_address): domain is the endpoint hostname agents call,
proxy_address the declared cluster address of the proxy serving it. The
serving shape is the shape of the pin -- self-addressed (domain ==
proxy_address, a proxy dedicated to the account) or labeled (domain one
label beneath a shared cluster's address) -- so no mode flag or config
exists anywhere, and any mix of shapes coexists per account on one
deployment.

Bootstrap becomes an explicit POST /api/agent-network/settings taking
exactly one of proxy_address (the server allocates an adjective-noun
label beneath it) or endpoint (claimed verbatim, address-first). The
identity fields leave the PUT schema entirely -- immutability by shape
rather than by runtime rejection -- and provider create loses
bootstrap_cluster and all settings side effects, which also retires two
latent bugs: the dashboard's unsorted [0] free-domain pick making a
permanent decision, and bootstrap failures swallowed at debug level
inside a 200 provider create.

The global unique index moves from the label to the full hostname --
the actual invariant. Labels may repeat across clusters again, and
self-addressed rows (no label) cannot collide on an empty string. The
reverse lookup becomes a point query on that index, deleting the
clusterFromDomain suffix heuristic, and the synthesizer stamps
ProxyCluster from the settings row as a field read. A pre-AutoMigrate
migration backfills existing rows (domain = subdomain.cluster,
proxy_address = cluster) and drops the legacy columns, failing loudly
on rows with no identity to derive.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

management/server/agentnetwork_realstack_test.go

@@ -92,6 +92,9 @@ func TestAgentNetwork_ProviderCRUD_FansOutToProxyAndClientPeers(t *testing.T) {
 	// UpdateAccountPeers, which is the path under test.
 	agentMgr := agentnetwork.NewManager(am.Store, permissions.NewManager(am.Store), am, nil)
 
+	_, err = agentMgr.CreateSettings(ctx, adminUserID, agenttypes.DefaultSettings(accountID), clusterAddr, "")
+	require.NoError(t, err, "CreateSettings must bootstrap the endpoint")
+
 	provider, err := agentMgr.CreateProvider(ctx, adminUserID, &agenttypes.Provider{
 		AccountID:   accountID,
 		ProviderID:  "openai_api",
@@ -100,7 +103,7 @@ func TestAgentNetwork_ProviderCRUD_FansOutToProxyAndClientPeers(t *testing.T) {
 		APIKey:      "sk-test-key",
 		Enabled:     true,
 		Models:      []agenttypes.ProviderModel{{ID: "gpt-5.4"}},
-	}, clusterAddr)
+	})
 	require.NoError(t, err, "CreateProvider must succeed")
 
 	policy, err := agentMgr.CreatePolicy(ctx, adminUserID, &agenttypes.Policy{