feat(brahman-handshake): ListSessions endpoint + cliente + UI broker-explorer

Iter 20. Nuevo flujo end-to-end para observabilidad: cualquier módulo
conectado puede pedir al broker la lista de sesiones activas y mostrar
labels + flows in/out por cada una.

brahman-handshake/messages:
- Frame::ListSessions(ListSessions{session}) → Frame::SessionList(SessionList{entries}).
- SessionEntry: session, label, schema_version, outputs, inputs, conscious.

brahman-handshake/server:
- run_post_handshake pasa SessionRegistry a handle_inbound_frame.
- build_session_list helper proyecta el snapshot bajo lock.
- Validación session_id mismatched → Unauthorized.

brahman-handshake/client:
- Client::list_sessions() async, drena MatchEvents intermedios al
  pending_events buffer, mismo patrón que ping().

brahman-sidecar/discovery:
- list_sessions / list_sessions_blocking arman Card observer mínima,
  piden, Farewell.

brahman-broker-explorer:
- Poll-tick agrega list_sessions_blocking cuando broker está UP*.
- stat_card "Sesiones activas" con count + items ordenados por Ulid:
  label · in:[flows] out:[flows]  (wit)?.

Test list_sessions_returns_currently_registered: 3 clientes
conectados, observer pide list, verifica labels + schema_version
+ conscious=false. 24 handshake tests + sidecar + broker-explorer
verde.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/shared/brahman-sidecar/src/lib.rs

@@ -18,7 +18,8 @@
 
 pub mod discovery;
 pub use discovery::{
-    await_provider, await_provider_blocking, build_consumer_card, ConsumerError,
+    await_provider, await_provider_blocking, build_consumer_card, list_sessions,
+    list_sessions_blocking, ConsumerError,
 };
 
 use std::collections::HashMap;