feat(discord): voice-channel mixer — ambient idle bed + verbal acks that overlap TTS (#39659)

* feat(discord): voice-channel mixer — ambient idle bed + verbal acks that overlap TTS

Discord voice mode can now feel conversational: the bot speaks a short
acknowledgement before it starts working, and a subtle ambient 'thinking' bed
plays underneath while tools run, ducking under speech and swelling back — the
Grok-voice-mode feel.

discord.py plays only one audio stream per voice connection, so this adds a
software mixer (VoiceMixer, a discord.AudioSource) installed once per guild on
join. It sums an ambient loop, verbal acks, and TTS replies into that single
20ms/48kHz/stereo stream (numpy int16 add + clip), so they overlap instead of
stop-and-swap. Speech ducks the ambient gain down and releases it smoothly.

- plugins/platforms/discord/voice_mixer.py: VoiceMixer + MixerChild (gain,
  loop, fade, duck/release), decode_to_pcm (ffmpeg), synth_ambient_pcm (no
  asset needed — synthesised pad).
- adapter: install mixer on join, tear down on leave, route
  play_in_voice_channel through the mixer (legacy one-shot path kept as
  fallback), play_ack_in_voice, voice_mixer_active. Defensive getattr for the
  object.__new__ test helpers.
- gateway/run.py: tool_start_callback fires a one-time verbal ack on the first
  tool call of a turn when in a voice channel (independent of the text
  tool-progress gate). No system-prompt or message-flow changes.
- config: discord.voice_fx.* (OFF by default; ambient/duck/speech gains, ack
  phrases). All in config.yaml, not .env.
- docs + tests (mixer unit + adapter integration).

Verified: 19 new tests pass, existing voice suite green (2 pre-existing
davey-module env failures unchanged), and a real-mixer E2E confirms ambient
streams, TTS overlaps it, acks layer in, and teardown is clean.

* fix(discord): make voice mixer numpy import lazy (numpy is voice-extra-only)

numpy ships in the optional 'voice' extra, not [all,dev], so a module-level
'import numpy' broke CI test collection (and would break the always-imported
Discord adapter on any install without the voice extra). Defer numpy to the
functions that actually mix audio via _require_numpy(); guard the test module
with pytest.importorskip('numpy').

hermes_cli/config.py

@@ -1841,6 +1841,28 @@ DEFAULT_CONFIG = {
         # real memory cost. Default 32 MiB matches the historical hardcoded
         # cap. Set to 0 for no cap. Env override: DISCORD_MAX_ATTACHMENT_BYTES.
         "max_attachment_bytes": 33554432,
+        # Voice-channel audio effects (the continuous mixer). OFF by default.
+        # When enabled, the bot installs a software mixer on the outgoing voice
+        # stream so a low ambient "thinking" bed, verbal acknowledgements, and
+        # TTS replies can OVERLAP (ducking the ambient under speech) instead of
+        # stop-and-swap — the Grok-voice-mode feel. discord.py ships no mixer;
+        # this is implemented in plugins/platforms/discord/voice_mixer.py.
+        "voice_fx": {
+            "enabled": False,         # master switch for the mixer subsystem
+            "ambient_enabled": True,  # play the idle "thinking" bed while tools run
+            "ambient_path": "",       # custom loop audio file; "" = synthesised pad
+            "ambient_gain": 0.18,     # idle bed loudness, 0.0–1.0
+            "duck_gain": 0.06,        # ambient loudness while speech plays
+            "speech_gain": 1.0,       # TTS / ack loudness, 0.0–1.0
+            "ack_enabled": True,      # speak a short phrase before the first tool call
+            "ack_phrases": [          # picked at random; set [] to disable phrases
+                "Let me look into that.",
+                "One moment.",
+                "Checking on that now.",
+                "Give me a sec.",
+                "On it.",
+            ],
+        },
     },
 
     # WhatsApp platform settings (gateway mode)