feat: rename ALPN prefix from alknet/ to alk/ (v0.1.1)

- CHANNELS_ALPN: b"alknet/channels" → b"alk/channels"
- CallAdapter::alpn(): b"alknet/call" → b"alk/call"
- derive_alpn_from_op_name: alknet/ prefix → alk/ prefix
- All ALPN string literals in src/ and docs/ updated
- ADR-004 amended with prefix rename rationale
- AGENTS.md, README.md updated
- Version bumped to 0.1.1

Review: docs/reviews/003-alpn-prefix-rename.md

Verification:
- cargo test: 542 passed, 0 failed
- cargo clippy --all-targets -- -D warnings: clean
- cargo fmt --check: clean
- cargo doc --no-deps: clean

docs/architecture/decisions/009-bistream-as-the-handler-leaf.md

@@ -345,7 +345,7 @@ pump halves, the same idiom it would use over `TcpStream`.
 ### `BiStream` over WebSocket enables "VPN-like without being a VPN" in v1
 
 The `webtransport.md` spec describes the "VPN-like without being a VPN"
-path: a browser opens a WebTransport session to `/alknet/ssh`, the h3
+path: a browser opens a WebTransport session to `/alk/ssh`, the h3
 handler hands each bidi stream to `SshAdapter::handle` as a
 `Connection`, the browser's WASM SSH parser speaks SSH over the
 stream. WebTransport is deferred per ADR-044.