Add channel-2 #command auto-responder (v0.18.0)

Event-driven CommandService dispatches exact case-sensitive #command
matches on COMMAND_CHANNEL (default 2) through a single-registration-point
registry; first command #ping -> #pong!. Replies ride COMMAND_CHANNEL:
directed DM back to the sender when present, else broadcast. Failing
handler/send only logs a warn.

DI: CommandModule's factory injects MeshService and attaches itself via
MeshService.attachCommands() — a constructor-level cycle leaves one side
null in NestJS 12's injector (verified), so no forwardRef is used.

Tests 106 -> 121; docs synced (AGENTS.md, MESHTASTIC.md, README).

src/app.module.ts

@@ -1,9 +1,10 @@
 import { Module } from '@nestjs/common';
 import { BatteryAlertModule } from './battery/battery.module.js';
+import { CommandModule } from './commands/command.module.js';
 import { MeshModule } from './mesh/mesh.module.js';
 import { WeatherModule } from './weather/weather.module.js';
 
 @Module({
-  imports: [MeshModule, WeatherModule, BatteryAlertModule],
+  imports: [MeshModule, WeatherModule, BatteryAlertModule, CommandModule],
 })
 export class AppModule {}