fix: verify SSH host keys, support ssh-agent, fix remote dir walk bugs

- Replace ssh.InsecureIgnoreHostKey() with TOFU verification against
  ~/.ssh/known_hosts (accept-new for unknown hosts, hard reject on a
  changed key) to close a MITM hole on every SFTP connection.
- Actually connect to a running ssh-agent (SSH_AUTH_SOCK) for auth
  instead of only scanning default key files on disk; give a clear
  error when an explicit IdentityFile is passphrase-protected.
- Fix SSHClient.walk()/DirectorySize: the internal filepathSkipDir
  sentinel leaked out as a real error on stat/ReadDir failures instead
  of being swallowed, aborting size calculation on the first
  unreadable subdirectory instead of skipping it.
- Skip symlink-to-directory entries in remote directory copies instead
  of failing the whole transfer trying to Open() them as regular files.
- Consolidate 5 duplicated sudo-aware home-dir lookups into
  internal/homedir, adding a missing os.Geteuid()==0 check before
  trusting SUDO_USER.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

vendor/modules.txt

@@ -96,7 +96,9 @@ golang.org/x/crypto/curve25519
 golang.org/x/crypto/internal/alias
 golang.org/x/crypto/internal/poly1305
 golang.org/x/crypto/ssh
+golang.org/x/crypto/ssh/agent
 golang.org/x/crypto/ssh/internal/bcrypt_pbkdf
+golang.org/x/crypto/ssh/knownhosts
 # golang.org/x/sys v0.43.0
 ## explicit; go 1.25.0
 golang.org/x/sys/cpu