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.

completions/pos.bash

@@ -30,6 +30,7 @@ _pos_subcmds[ai-gemini]="ask chat models sessions capture"
 _pos_subcmds[ai-openrouter]="ask chat sessions capture"
 _pos_subcmds[communication-matrix-sender]="send test login"
 _pos_subcmds[communication-scrcpy]="devices record tcpip connect push pull screenshot info"
+_pos_subcmds[communication-telegram-listener]="prefix"
 _pos_subcmds[communication-telegram-sender]="send test"
 _pos_subcmds[docker-compose]="ls installed up down restart logs update config menu"
 _pos_subcmds[docker-vbox]="create enter stop start rm ls menu"