feat(gateway): voice command service

Recognize simple device commands in the transcript and act on them without
a Tatlock round-trip. commands.match() maps volume up/down, mute/unmute,
"set volume to N", and "goes to eleven"/max to a "command" message sent
straight to the device; the utterance never reaches the LLM. Matching is
deliberately precise so real requests ("set an alarm for a quarter to
eleven") are not hijacked. Adds the "command" message to the device
protocol in docs/architecture.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LKPbR6DY2JygHbyLjxm7Uu

docs/architecture.md

@@ -266,8 +266,17 @@ gateway → device:  {"type": "reply_text", "text": "..."}
 gateway → device:  {"type": "audio_start", "sample_rate": 16000}
 gateway → device:  <binary PCM frames>   (may arrive sentence-by-sentence; play as a stream)
 gateway → device:  {"type": "audio_end"}
+
+gateway → device:  {"type": "command", "action": "volume_up"}   (LLM-bypass; see below)
 ```
 
+`command` (gateway → device) is an **alternative to the reply path**: when the
+gateway recognizes a simple device command in the transcript (volume/mute), it
+sends a `command` instead of calling Tatlock — no `reply_text`/audio — then returns
+to `idle`. Actions: `volume_up`, `volume_down`, `mute`, `unmute`, and `volume_set`
+with an extra `"level"` field (0–11, the on-device volume scale). Matched by the
+gateway's `commands.py`; applied on the device in `gw_client.c` → `face.c`.
+
 Planned additions (documented before implemented, here first):
 
 - `reply_delta` (gateway → device): incremental reply text for on-screen streaming while