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>

internal/homedir/homedir.go

@@ -0,0 +1,26 @@
+// Package homedir resolves the real user's home directory, including when
+// the process is running under sudo.
+package homedir
+
+import (
+	"os"
+	"os/user"
+)
+
+// Dir returns the real user's home directory. os.UserHomeDir() resolves to
+// /root when the process is running as root via sudo, which points config,
+// SSH, and trash paths at the wrong place. When the effective UID is 0 and
+// SUDO_USER is set, it resolves the invoking user's home instead.
+func Dir() string {
+	if os.Geteuid() == 0 {
+		if sudoUser := os.Getenv("SUDO_USER"); sudoUser != "" {
+			if u, err := user.Lookup(sudoUser); err == nil {
+				return u.HomeDir
+			}
+		}
+	}
+	if h, err := os.UserHomeDir(); err == nil {
+		return h
+	}
+	return ""
+}