refactor: rename telegram tool to pos-communication-telegram-sender; drop --send legacy alias

The --send flag duplicated the send subcommand — completion suggested both.
Now 'pos communication telegram <TAB>' completes to just 'sender' and
'listener'. send is the single canonical action.

- git mv bin/pos-communication-telegram bin/pos-communication-telegram-sender
- remove --send branch + POS_FLAGS entry; add --markdown to completion
- lib/notify.sh: notify_sender_name() maps platform telegram -> telegram-sender
  (notify_send otherwise looks for bin/pos-communication-telegram)
- pos-system-health --send 'sent:' check + pos-entertainment-send use the new
  name and the send subcommand
- completion: keys with nested tools but no direct tool complete to the group
  suffixes (sender/listener); --type/--parse-mode value completion moved to
  the new word positions
- docs updated (POS.md, AGENT_Context, DEV.md, SCRIPTS.md, notify.env,
  HOWTO.md, postinstall.sh); removed phantom webhook/log/broadcast subcommands
  from howto/communication.md

config/notify.env

@@ -4,5 +4,7 @@
 # them). Each platform must have a bin/pos-communication-<platform> tool that
 # implements the sender contract:
 #   pos-communication-<platform> send <value> [--markdown]
+# The telegram platform key maps to pos-communication-telegram-sender
+# (via notify_sender_name() in lib/notify.sh).
 # Default: telegram
 #NOTIFY_PLATFORM=telegram,matrix