Fix ApproveFingerprint: extract real host key via SSH instead of writing fingerprint SHA256 to known_hosts

The old ApproveFingerprint passed the SHA256 fingerprint string to
AddKnownHost which expected authorized_key format, causing known_hosts
entries to be corrupted and subsequent SSH connections (including shutdown)
to fail with "host key not found".

Changes:
- dialSSH now returns (conn, fingerprint, pubKey, error) with the raw
  ssh.PublicKey captured from the server
- New ConnectForApproval() wraps dialSSH with strictHostKeyChecking=false
  for the approval handshake
- ApproveFingerprint now opens an SSH connection to the host (non-strict),
  captures the real public key, and writes it in authorized_keys format
  to known_hosts via AddKnownHost
- shutdown.go updated to handle the new 4-value dialSSH return
- Supports optional host_key field in request body for direct key submission

internal/sshmanager/shutdown.go

@@ -14,7 +14,7 @@ type ShutdownResult struct {
 }
 
 func RunRemoteCommand(ctx context.Context, host string, port int, user, privKeyPath, knownHostsPath string, strictHostKeyChecking bool, command string) (*ShutdownResult, error) {
-	conn, _, err := dialSSH(ctx, host, port, user, privKeyPath, knownHostsPath, strictHostKeyChecking)
+	conn, _, _, err := dialSSH(ctx, host, port, user, privKeyPath, knownHostsPath, strictHostKeyChecking)
 	if err != nil {
 		return &ShutdownResult{Success: false, Error: err.Error()}, nil
 	}