feat: save onion addresses to file and .env on host

- entrypoint.sh: background process writes onion-hosts.txt with SSH_ONION and ADMIN_ONION
- docker-compose.yml: bind mount tor-proxy/hosts for onion address persistence on host
- tor-proxy/get-onions.sh: reads onion addresses and updates .env with ADMIN_URL, SSH_ONION, ADMIN_ONION
- .gitignore: exclude tor-proxy/hosts/onion-hosts.txt (secret)
- tor-proxy/hosts/.gitkeep: ensure directory exists in git

.gitignore

@@ -37,6 +37,9 @@ AGENTS.md
 # Local workspace / worktrees
 .work/
 
+# Tor onion addresses (secret)
+tor-proxy/hosts/onion-hosts.txt
+
 # Python cache
 __pycache__/
 *.pyc