feat: configurable AI-bridge trigger word for telegram listener

The listener's "ai " bridge prefix was hard-coded. Messages starting
with <prefix> + space (case-insensitive, literal match) are now
forwarded to Gemini; default stays "ai".

- TELEGRAM_AI_PREFIX in telegram.env (default ai), hot-reloaded per
  message like the command map — no daemon restart needed
- New 'prefix' verb: pos communication telegram listener prefix [word]
  (validated [A-Za-z0-9][A-Za-z0-9_-]*; writes telegram.env chmod 600)
- Field added to the telegram # POS_CONFIG: scope (sender header) so
  'pos config telegram' edits it too
- --status shows the current prefix; usage + POS_SUBCMDS: prefix
  (completions regenerated)
- Matching via scoped nocasematch + quoted-literal =~ prefix;
  ai_bridge_prefix() precedence: env file > env var > default ai

Verified: routing harness (default/custom/case-insensitive/reset/
fallback/unknown-command) green, CLI verb tests, dispatch smoke,
pos config render, gates 0 FAIL 0 WARN.

bin/pos-communication-telegram-sender

@@ -3,7 +3,7 @@ set -euo pipefail
 # POS: communication telegram-sender — Send Telegram messages/files/links/stickers via Bot API (send, test)
 # POS_FLAGS: --type --caption --parse-mode --no-preview --token --chat-id --markdown
 # POS_SUBCMDS: send test
-# POS_CONFIG: telegram | telegram.env | TELEGRAM_BOT_TOKEN=secret:Bot token from @BotFather | TELEGRAM_CHAT_ID=digits:Numeric chat id from @userinfobot
+# POS_CONFIG: telegram | telegram.env | TELEGRAM_BOT_TOKEN=secret:Bot token from @BotFather | TELEGRAM_CHAT_ID=digits:Numeric chat id from @userinfobot | TELEGRAM_AI_PREFIX=:AI-bridge trigger word in the telegram listener (default ai)::ai
 
 CONFIG_DIR="${CONFIG_DIR:-${XDG_CONFIG_HOME:-$HOME/.config}/linux_post_install}"
 CONFIG_FILE="$CONFIG_DIR/telegram.env"