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/042-hub-relay-translate-not-forward.md

@@ -91,8 +91,8 @@ spoke leg with `spoke_id` in the payload. The relay does not touch
 | Spoke leg | `ChannelsAdapter` + `CallAdapter` (same) |
 | Relay | Per-channel byte-forward tasks with `channel_id` rewrite |
 
-The hub never runs a handler for `alknet/tty`, `alknet/ssh`, or
-`alknet/tunnel`. It runs `alknet/channels` (the relay) and `alknet/call`
+The hub never runs a handler for `alk/tty`, `alk/ssh`, or
+`alk/tunnel`. It runs `alk/channels` (the relay) and `alk/call`
 (for its own hub-level operations + translation). The endpoints at each end
 do the protocol work.
 
@@ -102,7 +102,7 @@ do the protocol work.
   registry / ownership store (ADR-011), queried via call operations on
   channel 0. Channels doesn't touch this.
 - **ACL at the hub:** does this browser's identity have `channel:open` scope
-  for `alknet/ssh` to `spoke-X`? `AccessControl::check` on `channel/open`,
+  for `alk/ssh` to `spoke-X`? `AccessControl::check` on `channel/open`,
   run by the hub's `CallAdapter` before it forwards. Channels doesn't touch
   this.
 - **Relay lifecycle:** when a browser disconnects, the hub tears down the
@@ -168,7 +168,7 @@ auth path. The `channel_id` mapping strategy (`HashMap` per pair) is two-way
 - ADR-034: outgoing-only X.509 and the three peer roles (browser identity
   resolution)
 - ADR-011: dynamic resource ownership (the ownership store the hub queries)
-- ADR-036: channel 0 is pre-negotiated `alknet/call` (what the hub
+- ADR-036: channel 0 is pre-negotiated `alk/call` (what the hub
   terminates on each leg)
 - ADR-037: channel lifecycle operations (what the hub translates)
 - ADR-039: ChannelsAdapter and ChannelManager (the interface the relay uses)