feat: pos ai gemini — CLI + Telegram AI chat (ask, chat, models)

- bin/pos-ai-gemini: one-shot ask (stdout-only, pipe-friendly), interactive
  multi-turn chat REPL (q/Ctrl+C, /reset), models list; --model override,
  default gemini-2.5-flash; key via x-goog-api-key header, never printed.
- Config scope 'ai' (AI_GEMINI_API_KEY secret, AI_GEMINI_MODEL) in
  ~/.config/linux_post_install/ai.env via 'pos config ai'; config/ai.env
  template installed no-clobber by postinstall.
- Telegram listener: non-command text starting with 'ai ' (case-insensitive)
  is answered by Gemini via 'pos ai gemini ask'; owner-chat only, errors
  reply with the pos config ai hint. Future intents slot in as case arms.
- ai-gemini added to INTERACTIVE_CMDS (chat reads stdin).
- Docs: POS.md ai section + listener bridge, howto/ai.md, HOWTO/README
  index rows, bin/pos usage example; make gen refreshed context/completions.

postinstall.sh

@@ -37,7 +37,7 @@ fi
 # ── system + notify config templates ───────────────────────────
 # Copied only if the user has not already created their own (no clobber).
 run mkdir -p "$ENT_DIR"
-for tpl in system.env notify.env; do
+for tpl in system.env notify.env ai.env; do
     if [ -f "config/$tpl" ]; then
         if [ -f "$ENT_DIR/$tpl" ]; then
             log "$tpl already exists, keeping it"