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>

CLAUDE.md

@@ -102,6 +102,31 @@ This will:
 
 **No HAProxy reload required!**
 
+### Pool Sharing
+
+Multiple domains can share the same pool/backend servers using `share_with`:
+
+```bash
+# First domain gets its own pool
+haproxy_add_domain("example.com", "10.0.0.1")
+# Creates: example.com → pool_5
+
+# Additional domains share the same pool
+haproxy_add_domain("www.example.com", share_with="example.com")
+haproxy_add_domain("api.example.com", share_with="example.com")
+# Both use: pool_5 (same backend servers)
+```
+
+**Benefits:**
+- Saves pool slots (100 pools can serve unlimited domains)
+- Multiple domains → same backend servers
+- Shared domains stored as `_shares` reference in `servers.json`
+
+**Behavior:**
+- Shared domains cannot specify `ip` (use existing servers)
+- Removing a shared domain keeps servers intact
+- Removing the last domain using a pool clears the servers
+
 ### Backend Configuration
 - Backends always use HTTP (port 80 or custom)
 - SSL/TLS termination happens at HAProxy frontend
@@ -247,7 +272,7 @@ Returns backend server status for a specific domain:
 | Tool | Description |
 |------|-------------|
 | `haproxy_list_domains` | List domains (use `include_wildcards=true` for wildcards) |
-| `haproxy_add_domain` | Add domain to available pool (no reload), supports IPv6 |
+| `haproxy_add_domain` | Add domain to pool (no reload), supports `share_with` for pool sharing |
 | `haproxy_remove_domain` | Remove domain from pool (no reload) |
 
 ### Server Management