feat: add pos usb server (USB Redirector control); fix doc/code drift

- new bin/pos-usb-server: flag-style control of usbsrv (--ls, --share,
  --unshare, --auto-share, --callback, --close-callback, --auto-connect,
  --disconnect, --nickname, --timeout, --port, --info, --version);
  interactive prompts fall back to args; command -v usbsrv guard
- wire into bin/pos: usb category, example, INTERACTIVE_CMDS
- completions: usb-server flag completion
- docs: POS.md usb section, AGENT_Context tree/dispatch/table, README
- fix: add pos-network-hotspot to no-common.sh list (was omitted)
- fix: stale pos-docker-compose line count 317 -> 363
- fix: DEV.md/AGENT_Context note non-apt deps (command -v guard, not PACKAGES)
- fix: document INTERACTIVE_CMDS all-or-nothing per-script trade-off
- fix: usage() shows docker health (was missing from cheat-sheet)

completions/pos.bash

@@ -73,6 +73,10 @@ _pos() {
         COMPREPLY=($(compgen -W "$names" -- "$cur"))
     }
 
+    _pos_complete_usb_server_flags() {
+        COMPREPLY=($(compgen -W "--ls --ls-shared --share --unshare --auto-share --callback --close-callback --auto-connect --disconnect --nickname --timeout --port --info --version --help" -- "$cur"))
+    }
+
     # ── Dispatch ───────────────────────────────────────────────
     case "${#words[@]}" in
         2)
@@ -96,6 +100,9 @@ _pos() {
                 docker-vbox)
                     _pos_complete_docker_vbox_cmds
                     ;;
+                usb-server)
+                    _pos_complete_usb_server_flags
+                    ;;
             esac
             ;;
         5)