Add #help command — short single-message list of all commands (v0.21.6)

Owner request: 'now add #help with short list of all command'. Registered in
the CommandService constructor next to #ping; returns a SINGLE multi-line
string (one packet, ~203 chars) — not an array, which would drip-feed one
spaced packet per line through the v0.21.5 MESH_MULTI_SEND_DELAY_MS loop.

Also hardened the v0.21.5 multi-send-spacing spec: its wall-clock assertion
('first send < 8 ms after t0') flaked under full-suite load — replaced with
fake timers, which assert deterministically that line 1 is out before any time
elapses and lines 2-3 land exactly one delay apart.

src/commands/command.service.ts

@@ -43,6 +43,17 @@ export class CommandService {
     rss?: RssService,
   ) {
     this.register('ping', () => '#pong!');
+    // single multi-line string = ONE packet — not an array, which would drip-feed one spaced packet per line
+    this.register('help', () =>
+      [
+        '#ping - alive check',
+        '#rss help - rss subcommands',
+        '#rss add <url> <name> - add feed with AI tags',
+        '#rss remove <name> - drop feed',
+        '#rss get <name|#tag> - news summary',
+        '#rss sources / #rss tags - list feeds/tags',
+      ].join('\n'),
+    );
     if (rss) this.registerPrefix('rss', (ctx) => rss.handle(ctx));
   }