docs: rewrite README with structured overview and quick start guide

Replaces the minimal project description with a comprehensive README
including a component overview table, quick start instructions, common
Ansible operations, and links to detailed documentation. Aligns with
Red Panda Approval™ standards.

terraform/variables.tf

@@ -0,0 +1,41 @@
+variable "project_name" {
+  description = "Name of the Incus project for sandbox environment"
+  type        = string
+  default     = "agathos"
+}
+
+variable "profile_name" {
+  description = "Name of the Incus profile for sandbox hosts"
+  type        = string
+  default     = "sandbox"
+}
+
+variable "network_name" {
+  description = "Name of the network bridge"
+  type        = string
+  default     = "incusbr0"
+}
+
+variable "storage_pool" {
+  description = "Name of the storage pool"
+  type        = string
+  default     = "default"
+}
+
+variable "system_user" {
+  description = "System user name for sandbox hosts"
+  type        = string
+  default     = "robert"
+}
+
+variable "user_uid" {
+  description = "System user UID"
+  type        = number
+  default     = 1000
+}
+
+variable "ssh_key_path" {
+  description = "Path to SSH authorized keys file"
+  type        = string
+  default     = "~/.ssh/authorized_keys"
+}