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.

ansible/sandbox_down.yml

@@ -0,0 +1,27 @@
+---
+- name: Stop Agathos Sandbox Uranian Hosts
+  hosts: localhost
+  gather_facts: false
+  vars:
+    uranian_hosts:
+      - oberon
+      - portia
+      - ariel
+      - puck
+      - miranda
+      - sycorax
+      - prospero
+      - rosalind
+      - titania
+  tasks:
+    - name: Stop Uranian host containers
+      ansible.builtin.command:
+        cmd: incus stop {{ item }} --project agathos
+      loop: "{{ uranian_hosts }}"
+      register: stop_result
+      failed_when: stop_result.rc != 0 and 'not running' not in stop_result.stderr
+      changed_when: stop_result.rc == 0
+      
+    - name: Display completion message
+      ansible.builtin.debug:
+        msg: "🐾 Uranian hosts stopped gracefully"