test(slice-2): integration test + Python echo brain + LEARNING pointers

- Integration test: smoke test against in-process EchoServer (hello
  handshake round-trip). Full end-to-end + reconnect-path tests adapt
  to slice-1's existing integration-test harness.
- examples/echo_brain/: Python reference brain (~80 lines, websockets lib).
  README documents runtime posture + why a Python brain complements the
  Rust one (language-agnosticism vs wire-types-reusability).
- LEARNING.md: 5 new pointers (mpsc/oneshot, VecDeque ring, async WS,
  Box<dyn Trait> field widening, zero-sized marker newtype).

Spec ref: 2026-06-28-slice-2-agent-tap-design.md §8.4, §8.5 #7.

examples/echo_brain/README.md

@@ -0,0 +1,46 @@
+# Rutster Echo Brain (Python reference)
+
+The canonical foreign-language brain for slice-2. Speaks the [slice-2 tap wire
+protocol](../../docs/superpowers/specs/2026-06-28-slice-2-agent-tap-design.md) (§3):
+versioned JSON events over `ws://` text frames with base64-encoded
+little-endian PCM.
+
+## Why a Python brain?
+
+The architecture (ARCHITECTURE.md §"Agent tap") names "a Python script" as the
+canonical brain persona. This example proves the wire format is language-agnostic:
+the core's `rutster-tap` Rust types serialize/deserialize to exactly what
+`json.loads` + `websockets` produces. The Rust echo brain (`crates/rutster-tap-echo`)
+proves wire-types reusability from outside the core; this Python brain proves
+language-agnosticism.
+
+## Run
+
+```bash
+pip install -r examples/echo_brain/requirements.txt
+python examples/echo_brain/echo_brain.py
+# [echo_brain] listening on ws://127.0.0.1:8081/echo
+```
+
+Then in another terminal:
+
+```bash
+cargo run  # the core dials out to ws://127.0.0.1:8081/echo by default
+```
+
+## Not in CI
+
+This script is **not** run by CI. The Rust `rutster-tap-echo` crate covers
+the in-process test surface for the integration test; this Python brain is
+the human-runnable demo + manual e2e test plan.
+
+## Protocol
+
+See [the spec](../../docs/superpowers/specs/2026-06-28-slice-2-agent-tap-design.md)
+§3 for the full wire protocol. Key behavior:
+
+- On `hello`: ack with `hello` (echo the `session_id`).
+- On `audio_in`: echo the same PCM back as `audio_out` (advisory — core
+  disposes).
+- On `bye` / `session_end`: close cleanly.
+- Stateless across reconnects (spec §5.3) — every `hello` starts fresh.