Drop audioop: request 16 kHz directly via Speaches sample_rate extension

Settles the Python version question: floor >=3.11, no ceiling.
Container moves to python:3.13-slim, CI tests on 3.13. The embedded
Piper fallback resamples with numpy (already present via [speech]).
Verified with a live round trip at 16 kHz.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

AGENTS.md

@@ -15,9 +15,11 @@ one repo:
   (Tatlock `/v1/chat/completions`) → TTS. Listens on port **8600**. STT/TTS models live
   in the shared **Speaches** container (live on port 8601, OpenAI-format API), not in
   the gateway image; `stt.py`/`tts.py` are pluggable backends (`speaches` default,
-  `embedded` fallback needing the `[speech]` extra). Gateway runs on **Python 3.12
-  exactly** — system python3 on tower-of-joy is 3.8, and `audioop` (used for TTS
-  resampling) is removed in 3.13.
+  `embedded` fallback needing the `[speech]` extra). Gateway needs **Python ≥ 3.11**
+  (no ceiling; the container runs 3.13) — but system python3 on tower-of-joy is 3.8,
+  so `make setup` explicitly uses `python3.12`. No local audio resampling in the
+  default path: the gateway requests 16 kHz output via Speaches' `sample_rate`
+  extension (verified live).
 
 The device and gateway speak a WebSocket protocol defined in `docs/architecture.md`.
 **That doc is the contract** — update it in the same change as any protocol edit on