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.

tests/test-hostkey-policy.bats

@@ -0,0 +1,7 @@
+#!/usr/bin/env bats
+
+# Scaffold for host key policy; to be implemented with a controlled SSH target or dry-run hooks.
+
+@test "BACKTUNNEL_HOSTKEY_POLICY scaffold" {
+  skip "TODO: verify policy propagation into ssh/sftp/sshfs invocations"
+}