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-umount.bats

@@ -0,0 +1,11 @@
+#!/usr/bin/env bats
+
+@test "backtunnel-umount shows usage with no args" {
+  run scripts/backtunnel-umount
+  [ "$status" -ne 0 ]
+  [[ "$output" == *"Usage:"* ]]
+}
+
+@test "backtunnel-umount integration (scaffold)" {
+  skip "TODO: create a temporary FUSE mount and verify unmount behavior"
+}