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.

DOC/howto/ai.md

@@ -6,8 +6,13 @@ Tools: `gemini` (`ask`, `chat`, `models`).
 | Tool | What it does |
 |------|--------------|
 | `pos ai gemini ask "<prompt>"` | One-shot answer to stdout (scriptable) |
+| `pos ai gemini ask --session <name> "…"` | Same, but remembers prior turns |
 | `pos ai gemini chat` | Interactive multi-turn conversation |
 | `pos ai gemini models` | List available model ids |
+| `pos ai gemini sessions` | List/clear persistent sessions (`reset <name>`) |
+
+Shared flags: `--model <id>` overrides the model; `--system "<text>"` adds a
+system instruction to every turn (kept out of the session file).
 
 ---
 
@@ -50,6 +55,14 @@ Set a different model per message:
 you:    ai --model gemini-2.5-flash explain a Raft consensus log
 ```
 
+### Telegram memory & formatting
+
+Each chat has its own persistent session (`telegram-<chat id>`), so the model
+remembers the conversation; `ai /reset` clears it. The listener passes a system
+prompt telling the model it is answering in a Telegram chat — so it uses emojis
+and stays lively — and strips markdown (`**x**`, backticks, `#`, links…) from
+the reply before sending it, since messages go out as plain text.
+
 ## Recipes
 
 - **Answer from a file:** `pos ai gemini ask "$(cat notes.txt)"`