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.

DOC/howto/ai.md

@@ -0,0 +1,92 @@
+# How-To: `pos ai`
+
+Chat with Google Gemini — from the terminal and through the Telegram bot.
+Tools: `gemini` (`ask`, `chat`, `models`).
+
+| Tool | What it does |
+|------|--------------|
+| `pos ai gemini ask "<prompt>"` | One-shot answer to stdout (scriptable) |
+| `pos ai gemini chat` | Interactive multi-turn conversation |
+| `pos ai gemini models` | List available model ids |
+
+---
+
+## First run — get a key, configure, chat
+
+1. Get a free API key from https://aistudio.google.com/apikey (requires a Google
+   account).
+2. Configure it (masked input):
+
+   ```bash
+   pos config ai          # enter AI_GEMINI_API_KEY
+   ```
+
+3. Test:
+
+   ```bash
+   pos ai gemini ask "Explain DNS in one line"
+   pos ai gemini models                 # verify the default model id is live
+   pos ai gemini chat                   # multi-turn conversation
+   ```
+
+`ai.env` lives at `~/.config/linux_post_install/ai.env` (chmod 600); `pos config ai`
+is the only place the key is written. The key is never printed by `pos`.
+
+## From the Telegram bot
+
+Once `pos ai gemini ask` works, any non-command message starting with `ai ` is
+answered by the model — no bot map entry needed:
+
+```
+you:    ai what is Nvidia
+bot:    NVIDIA is a company best known for GPUs...
+```
+
+The bridge lives in the Telegram listener's `handle_message` (it calls
+`pos ai gemini ask`); only the owner chat is served, so your key stays private.
+Set a different model per message:
+
+```
+you:    ai --model gemini-2.5-flash explain a Raft consensus log
+```
+
+## Recipes
+
+- **Answer from a file:** `pos ai gemini ask "$(cat notes.txt)"`
+- **Pipe into it:** `echo "fix this: $(cat error.log)" | pos ai gemini ask`
+- **Answer in a cron job:** `pos ai gemini ask "summarize today's git log" > /tmp/ai_digest.txt`
+- **Change the default model:**
+  ```bash
+  pos config ai          # set AI_GEMINI_MODEL, or:
+  AI_GEMINI_MODEL=gemini-2.5-flash pos ai gemini ask "hi"
+  ```
+
+## How it works
+
+- `ask` POSTs `contents:[{role:user, parts:[{text:"…"}]}]` to
+  `https://generativelanguage.googleapis.com/v1beta/models/<model>:generateContent`
+  with the key in the `x-goog-api-key` header, and prints
+  `.candidates[0].content.parts[].text` — nothing else.
+- `chat` keeps the whole conversation in memory as a growing `contents[]` array,
+  so later turns have earlier context. `/reset` drops it.
+- On a non-2xx response the API's `error.message` is shown and the exit code is
+  non-zero — so scripts can rely on `ask` failing loudly.
+
+## Troubleshooting
+
+- `ask` errors "No Gemini API key — run 'pos config ai'" → the key isn't set
+  (or `ai.env` isn't readable). Run `pos config ai`.
+- `API error 400` → the model id is wrong or the prompt is too long for the
+  model's context window; check `pos ai gemini models`.
+- `API error 429` → rate limit (free tier); wait and retry, or use a different
+  model.
+- Nothing in Telegram for `ai …` → the listener daemon must be running
+  (`pos communication telegram listener --status`); the bot token and owner
+  chat id must match `pos config telegram`.
+
+---
+
+## Related
+
+- Reference: [DOC/POS.md → ai](../POS.md#ai)
+- Telegram bridge context: [communication.md](communication.md)