feat: Add SSH remote execution for HAProxy on remote host

MCP server can now manage HAProxy running on a remote host via SSH.
When SSH_HOST env var is set, all file I/O and subprocess commands
(podman, acme.sh, openssl) are routed through SSH instead of local exec.

- Add ssh_ops.py module with remote_exec, run_command, file I/O helpers
- Modify file_ops.py to support remote reads/writes via SSH
- Update all tools (domains, certificates, health, configuration) for SSH
- Fix domains.py: replace direct fcntl usage with file_lock context manager
- Add openssh-client to Docker image for SSH connectivity
- Update k8s deployment with SSH env vars and SSH key secret mount

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

haproxy_mcp/tools/configuration.py

@@ -1,6 +1,5 @@
 """Configuration management tools for HAProxy MCP Server."""
 
-import subprocess
 import time
 
 from ..config import (
@@ -18,6 +17,7 @@ from ..file_ops import (
     get_domain_backend,
     get_backend_and_prefix,
 )
+from ..ssh_ops import run_command
 
 
 def restore_servers_from_config() -> int:
@@ -167,17 +167,17 @@ def register_config_tools(mcp):
             Validation result or error details
         """
         try:
-            result = subprocess.run(
+            result = run_command(
                 ["podman", "exec", HAPROXY_CONTAINER, "haproxy", "-c", "-f", "/usr/local/etc/haproxy/haproxy.cfg"],
-                capture_output=True, text=True, timeout=SUBPROCESS_TIMEOUT
+                timeout=SUBPROCESS_TIMEOUT,
             )
             if result.returncode == 0:
                 return "Configuration is valid"
             return f"Configuration errors:\n{result.stderr}"
-        except subprocess.TimeoutExpired:
+        except TimeoutError:
             return f"Error: Command timed out after {SUBPROCESS_TIMEOUT} seconds"
         except FileNotFoundError:
-            return "Error: podman command not found"
+            return "Error: ssh/podman command not found"
         except OSError as e:
             return f"Error: OS error: {e}"