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.

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## v0.21.6 — 2026-09-26 10:37
+
+- New `#help` command (owner request): replies with a short list of all available commands as ONE multi-line message (single packet, not one spaced packet per line) — `#ping`, the `#rss help/add/remove/get/sources/tags` family. Registered in the CommandService constructor next to `#ping`.
+- Tests: 173 → 174 — `#help` dispatches through respond() exactly once with text containing both `#ping` and `#rss`. (Also hardened the v0.21.5 multi-send-spacing spec, which flaked under full-suite load: it now uses fake timers to assert deterministically that line 1 goes out before any time elapses and lines 2-3 land exactly one delay apart.)
+
 ## v0.21.5 — 2026-09-26 10:26
 
 - Multi-line command replies are now spaced apart: `CommandService.onRx` waits `MESH_MULTI_SEND_DELAY_MS` (new env, default **5000** = 5 s, read at construction) between consecutive lines of an array reply — no delay before the first line. Production incident: `#rss get lublin` sent its items back-to-back within the same second and 3 of 4 were dropped in RF (the owner saw only the last one); each packet now gets time to land.