Add backtunnel-umount script, host key policy support, and extend TUI/CLI integration

Introduce `backtunnel-umount` as a portable unmount helper, preferring `fusermount3`, `fusermount`, or `umount`. Add `BACKTUNNEL_HOSTKEY_POLICY` for configurable host key handling in `backtunnel-share` and `backtunnel-access`. Update TUIs for remote folder prompts and mount point handling. Enhance bash completion for TUI commands with directory suggestions. Revamp terminal selection logic in `backtunnel-open-term` to prioritize modern emulators like wezterm. Extend tests with scaffolds for host key policy and unmount behavior. Update README with new scripts, workflows, features, and troubleshooting tips.

scripts/backtunnel-access

@@ -41,6 +41,13 @@ set -euo pipefail
 PORT=2222
 MOUNTPOINT="$HOME/remote-rssh"
 
+# Host key checking policy: env BACKTUNNEL_HOSTKEY_POLICY = yes|no|ask|accept-new (default: accept-new)
+HKP="${BACKTUNNEL_HOSTKEY_POLICY:-accept-new}"
+case "$HKP" in
+  yes|no|ask|accept-new) ;;
+  *) HKP="accept-new" ;;
+esac
+
 usage() {
   echo "Usage: $0 /path/to/folder from remoteuser:remotehost [-p PORT] [-m MOUNTPOINT]" >&2
   exit 1
@@ -151,7 +158,7 @@ if [[ -f "$HOME/.ssh/id_ed25519_backtunnel" ]]; then
   SFTP_ID_OPTS+=( -o IdentityFile="$HOME/.ssh/id_ed25519_backtunnel" -o IdentitiesOnly=yes )
 fi
 
-if ! ssh -o BatchMode=yes -o StrictHostKeyChecking=accept-new \
+if ! ssh -o BatchMode=yes -o StrictHostKeyChecking="$HKP" \
         -p "$PORT" "${SSH_IDENTITY_OPTS[@]}" "$REMOTE_USER@localhost" true 2>/dev/null; then  cat >&2 <<EOF
 ⚠️  Passwordless auth not set for $REMOTE_USER@localhost:$PORT.
     You can initialize a tunnel-only, SFTP-only key with:
@@ -164,14 +171,14 @@ fi
 echo "Checking remote path visibility via SFTP ..."
 
 # Purpose: quick sanity check that the target path is visible over SFTP before mounting.
-if ! sftp -q -P "$PORT" -o StrictHostKeyChecking=accept-new "${SFTP_ID_OPTS[@]}" \
+if ! sftp -q -P "$PORT" -o StrictHostKeyChecking="$HKP" "${SFTP_ID_OPTS[@]}" \
          "$REMOTE_USER@localhost" <<< "ls -1 \"$FOLDER\"" >/dev/null 2>&1; then
   echo "⚠️  Remote path '$FOLDER' not listable via SFTP. It may not exist or permissions deny access." >&2
   echo "    Proceeding to mount; sshfs may fail if the path is invalid." >&2
 fi
 
 # Build ssh command used by sshfs (adds keepalive/connect-timeout, identity if present).
-SSH_CMD="ssh -o ConnectTimeout=10 -o StrictHostKeyChecking=accept-new"
+SSH_CMD="ssh -o ConnectTimeout=10 -o StrictHostKeyChecking=$HKP"
 # If identity options are present, append them to SSH_CMD
 if [[ ${#SSH_IDENTITY_OPTS[@]} -gt 0 ]]; then
   # Join array safely
@@ -190,4 +197,4 @@ sshfs \
   -- "$REMOTE_USER@localhost:$FOLDER" "$MOUNTPOINT"
 
 echo "✅ Mounted at: $MOUNTPOINT"
-echo "To unmount:   fusermount -u \"$MOUNTPOINT\" || fusermount3 -u \"$MOUNTPOINT\""
+echo "To unmount:   backtunnel-umount \"$MOUNTPOINT\""