Fix: Always use explicit SSH key path for all SSH operations

CRITICAL FIX: SSH keys were not being auto-loaded, causing connection failures.

Changes:
- tools.py: SSH commands now include -i /var/lib/macha/.ssh/id_ed25519
- remote_monitor.py: Use explicit key path instead of sudo ssh
- system_discovery.py: Added explicit key path to all SSH calls
- system_prompt.txt: Document automatic SSH key loading
- DESIGN.md: Clarify CRITICAL requirement for explicit key paths

All SSH operations now explicitly specify:
  -i /var/lib/macha/.ssh/id_ed25519 -o StrictHostKeyChecking=no

This ensures Macha can reliably connect to remote hosts without
depending on SSH agent or automatic key discovery.

remote_monitor.py

@@ -36,9 +36,11 @@ class RemoteMonitor:
             (success, stdout, stderr)
         """
         try:
-            # Use sudo to run SSH as root (which has the keys)
+            # Use explicit SSH key path from macha user's home directory
+            ssh_key = "/var/lib/macha/.ssh/id_ed25519"
             ssh_cmd = [
-                "sudo", "ssh",
+                "ssh",
+                "-i", ssh_key,
                 "-o", "StrictHostKeyChecking=no",
                 "-o", "ConnectTimeout=10",
                 self.ssh_target,