feat: Add pool sharing for domains

Allow multiple domains to share the same backend pool using share_with parameter.
This saves pool slots when domains point to the same servers.

- Add share_with parameter to haproxy_add_domain
- Add helper functions for shared domain management
- Protect shared pools from being cleared on domain removal
- Update documentation with pool sharing examples

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

haproxy_mcp/file_ops.py

@@ -332,6 +332,63 @@ def remove_domain_from_config(domain: str) -> None:
             save_servers_config(config)
 
 
+def get_shared_domain(domain: str) -> Optional[str]:
+    """Get the parent domain that this domain shares a pool with.
+
+    Args:
+        domain: Domain name to check
+
+    Returns:
+        Parent domain name if sharing, None otherwise
+    """
+    config = load_servers_config()
+    domain_config = config.get(domain, {})
+    return domain_config.get("_shares")
+
+
+def add_shared_domain_to_config(domain: str, shares_with: str) -> None:
+    """Add a domain that shares a pool with another domain.
+
+    Args:
+        domain: New domain name
+        shares_with: Existing domain to share pool with
+    """
+    with file_lock(f"{SERVERS_FILE}.lock"):
+        config = load_servers_config()
+        config[domain] = {"_shares": shares_with}
+        save_servers_config(config)
+
+
+def get_domains_sharing_pool(pool: str) -> list[str]:
+    """Get all domains that use a specific pool.
+
+    Args:
+        pool: Pool name (e.g., 'pool_5')
+
+    Returns:
+        List of domain names using this pool
+    """
+    domains = []
+    for domain, backend in get_map_contents():
+        if backend == pool and not domain.startswith("."):
+            domains.append(domain)
+    return domains
+
+
+def is_shared_domain(domain: str) -> bool:
+    """Check if a domain is sharing another domain's pool.
+
+    Args:
+        domain: Domain name to check
+
+    Returns:
+        True if domain has _shares reference, False otherwise
+    """
+    config = load_servers_config()
+    domain_config = config.get(domain, {})
+    return "_shares" in domain_config
+
+
 # Certificate configuration functions
 
 def load_certs_config() -> list[str]: