feat: ai sessions + Telegram-friendly replies (--system, markdown strip)

pos ai gemini: --session gives ask/chat persistent memory
(~/.local/share/linux_post_install/ai/<name>.json, capped 40 turns),
new 'sessions' subcommand to list/clear; --system injects a Gemini
systemInstruction on every turn without storing it in the session file.
Telegram listener keeps one session per chat (telegram-<chat_id>), clears
on 'ai /reset', passes a Telegram-voice system prompt (emojis welcome),
and strips markdown from replies before sendMessage since messages are
sent as plain text. Docs: howto/ai.md flags/sessions/bridge behavior.

completions/pos.bash

@@ -3,7 +3,7 @@
 # Install: source this file in ~/.bashrc or place in /etc/bash_completion.d/
 # GEN:START posflags
 declare -A _pos_flags
-_pos_flags[ai-gemini]="--model"
+_pos_flags[ai-gemini]="--model --session --system"
 _pos_flags[communication-telegram-listener]="--enable --disable --status --sync-commands --run"
 _pos_flags[communication-telegram-sender]="--type --caption --parse-mode --no-preview --token --chat-id --markdown"
 _pos_flags[entertainment-send]="--print --markdown"
@@ -14,7 +14,7 @@ _pos_flags[usb-server]="--ls --ls-shared --share --unshare --auto-share --callba
 # GEN:END posflags
 # GEN:START possubcmds
 declare -A _pos_subcmds
-_pos_subcmds[ai-gemini]="ask chat models"
+_pos_subcmds[ai-gemini]="ask chat models sessions"
 _pos_subcmds[communication-telegram-sender]="send test"
 _pos_subcmds[docker-compose]="ls installed up down restart logs update config"
 _pos_subcmds[docker-vbox]="create enter stop start rm ls"