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/019-operation-registry-layering.md

@@ -17,9 +17,9 @@ as sharing one immutability argument:
 
 2. **The call protocol's `OperationRegistry`** (operation name →
    `HandlerRegistration`). This lives *inside* the `CallAdapter`, which is one
-   `ProtocolHandler` behind the single ALPN `alknet/call`. Adding an operation
+   `ProtocolHandler` behind the single ALPN `alk/call`. Adding an operation
    to the `OperationRegistry` does **not** touch the TLS `ServerConfig` — the
-   ALPN is already `alknet/call`, registered once at startup.
+   ALPN is already `alk/call`, registered once at startup.
 
 `operation-registry.md` stated the operation registry "is immutable after
 construction… consistent with OQ-04 and ADR-010." That inheritance was by