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).

README.md

@@ -121,6 +121,7 @@ Config lives in `.env` (gitignored) and is passed via compose environment — cr
 - `MESH_RECONNECT_MS` (default `5000`) — reconnect interval in ms
 - `MESH_SEND_CHANNEL` (default `3`) — channel index all sends target (`test`; never the primary channel)
 - `MESH_NOTIFY_CHANNEL` (default `3`) — comma-separated list of channel indices whose rx texts trigger the opencode notifier (see "Opencode notifier"; on ch0 only directed DMs notify, broadcasts ignored; synthetic injected messages, channel 0, are exempt and trigger it when `0` is in the list; messages from our own node are never notified)
+- `COMMAND_CHANNEL` (default `2`) — channel index of the auto-responder: exact `#command` texts on that channel get an automatic reply (directed back to the sender, else broadcast); currently only `#ping` → `#pong!`
 - `MESH_DB_PATH` (default `./data/mesh-bot.db`) — SQLite file backing the message history (parent dir created if missing; in Docker, mounted at `/app/data` via a named volume so it survives container recreation)
 - `MESH_POLL_MS` (default `1000`) — how often the poller re-upserts the message history into SQLite and prunes rows beyond the newest 1000
 - `PORT` (default `3000`) — HTTP listen port