Bump VERSION to 0.2.8, finalize changelog

Team admin settings audit (pass 1): remove dead BYOK/provider/persona
vestiges from team store, models, policy defaults, workflow stage UI,
and ICD tests. Fix stale chat_only stage mode in frontend to match
backend CHECK constraint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/models/models.go

@@ -78,15 +78,13 @@ type TeamMember struct {
 	Email       string `json:"email,omitempty"`
 	DisplayName string `json:"display_name,omitempty"`
 	Username    string `json:"username,omitempty"`
-	UserRole    string `json:"user_role,omitempty"`
 }
 
 // PLATFORM POLICIES
 
 var PolicyDefaults = map[string]string{
-	"allow_registration":   "true",
-	"default_user_active":  "false",
-	"allow_team_providers": "true",
+	"allow_registration":  "true",
+	"default_user_active": "false",
 }
 
 // HandleFromName generates a URL-safe @mention handle from a display name.