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

src/channels/mod.rs

@@ -1,7 +1,7 @@
 //! Channels protocol: N logical channels over one transport stream
 //! (ADR-034, amended by ADR-035 — no `stream_type` concept).
 //!
-//! Channel 0 is pre-negotiated as `alknet/call` (ADR-036); channels
+//! Channel 0 is pre-negotiated as `alk/call` (ADR-036); channels
 //! 1..N are opened dynamically via per-ALPN open ops on channel 0
 //! (ADR-047). The channels layer is a re-framing proxy: it converts
 //! between "one transport stream carrying N channels" (the wire) and
@@ -18,7 +18,7 @@
 //! - [`source`]: `ChannelBidiStreamSource` — implements
 //!   `BidiStreamSource` for a single channel (yield-once `accept_bi`).
 //! - [`adapter`]: `ChannelsAdapter` — `ProtocolHandler` for
-//!   `alknet/channels` (the demux loop).
+//!   `alk/channels` (the demux loop).
 //! - [`operations`]: `ChannelOperations` — registers `channel/close`,
 //!   `channel/control`, `channel/resources/subscribe` on the call
 //!   `OperationRegistry`; the per-ALPN open ops are registered by the