docs(slice-3): Python reference brain + LEARNING.md + README dev loop (spec §7.5)

examples/openai_realtime_brain/ -- the canonical foreign-language OpenAI
Realtime brain (Python, ~150 lines, websockets lib only). Not in CI
(zero-non-Rust-dev-deps dev loop per AGENTS.md). Mirrors the slice-2
echo_brain pattern and proves the protocol is language-agnostic.

Hand-rolled against the spec §4.2 mapping rather than using the openai
Python SDK: the SDK is unused by the wire-shape (both sides are plain
WSS), so dropping it removes a dep that the plan skeleton imported but
never called. session.update is sent with turn_detection=null (S4)
matching the Rust brain.

LEARNING.md gains 5 new pointers under a Slice 3 heading: async-trait
in trait objects, the translator pure-function layer, additive protocol
extension via #[serde(other)], the FOB-boundary side-channel mpsc, and
the WS subprotocol handshake (openai_client.rs).

README.md gains the Slice 3 dev loop section (mock mode without an
OpenAI key, real-OpenAI mode, and the Python brain alternative).

.gitignore learns about __pycache__/*.pyc (the Python brains aren't in
CI but byte-compiled artifacts should never be tracked).

README.md

@@ -19,6 +19,38 @@ cargo run
 Open <http://localhost:8080/> → click "Start call" → grant mic → hear yourself echo.
 Full walkthrough + troubleshooting: **[`docs/QUICKSTART.md`](docs/QUICKSTART.md)**.
 
+### Slice 3 dev loop — OpenAI Realtime brain
+
+The dev loop *without* real OpenAI credentials (no API key required):
+
+```
+cargo run -p rutster-brain-realtime --features=mock   # brain on :8082
+cargo run                                            # core on :8080
+```
+
+Open <http://localhost:8080/> → click "Start call" → speak → hear the
+mock-brain reply within ~250 ms (the mock echoes audio back, no real OpenAI
+RTT; this exercises the full brain→core audio round-trip + the new
+function_call dispatch path).
+
+With real OpenAI Realtime:
+
+```
+export OPENAI_API_KEY=sk-...    # or OPENAI_API_KEY_FILE=/var/secrets/openai
+cargo run -p rutster-brain-realtime
+cargo run
+```
+
+Speak → end-to-end speech-to-speech with OpenAI Realtime within ~700 ms
+(slice-1's 200 ms + tap round-trip + OpenAI latency + 100 ms playout buffer).
+
+For the foreign-language brain demo (Python, not in CI):
+
+```
+pip install -r examples/openai_realtime_brain/requirements.txt
+OPENAI_API_KEY=sk-... python examples/openai_realtime_brain/openai_realtime_brain.py
+```
+
 > **Status:** Slice 1 (WebRTC media loopback) is the active build target. The workspace is
 > landing task-by-task on the `slice-1-webrtc-loopback` branch. Design:
 > [`docs/superpowers/specs/2026-06-28-slice-1-webrtc-loopback-design.md`](docs/superpowers/specs/2026-06-28-slice-1-webrtc-loopback-design.md).