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/config/session.go

@@ -6,6 +6,8 @@ import (
 	"path/filepath"
 
 	toml "github.com/pelletier/go-toml/v2"
+
+	"vcom/internal/homedir"
 )
 
 // SessionState stores the UI state that should be restored on next launch.
@@ -24,9 +26,9 @@ type PaneSession struct {
 
 // DefaultSessionPath returns the path to the session file.
 func DefaultSessionPath() (string, error) {
-	homeDir, err := os.UserHomeDir()
-	if err != nil {
-		return "", fmt.Errorf("resolve home dir: %w", err)
+	homeDir := homedir.Dir()
+	if homeDir == "" {
+		return "", fmt.Errorf("cannot determine home directory")
 	}
 	xdgDir := os.Getenv("XDG_CONFIG_HOME")
 	if xdgDir == "" {