agent: unify listening and dialing with directed mutual measurements

Replace Probe with Agent across node runtime, Fleet contracts, backend state, API and storage. Report incoming sessions and outgoing link health through each Agent uplink, and retain directed link controls and retry.

Update protocol fixtures and end-to-end coverage for the unified model; accept requested cancellation during the Fleet timeout test cleanup.

backend/shared/MoShiro.Protocol/Ids/MoShiroIds.cs

@@ -3,14 +3,6 @@ using Fleet.Core;
 
 namespace MoShiro.Protocol;
 
-[JsonConverter(typeof(ProbeIdJsonConverter))]
-public readonly record struct ProbeId(string Value)
-{
-    public const string Prefix = "prb";
-
-    public override string ToString() => Value;
-}
-
 [JsonConverter(typeof(TaskIdJsonConverter))]
 public readonly record struct TaskId(string Value)
 {
@@ -42,13 +34,6 @@ public static class AgentIds
     public const string Prefix = "agt";
 }
 
-public sealed class ProbeIdJsonConverter : StringIdJsonConverter<ProbeId>
-{
-    protected override ProbeId Create(string value) => new(value);
-
-    protected override string GetValue(ProbeId id) => id.Value;
-}
-
 public sealed class TaskIdJsonConverter : StringIdJsonConverter<TaskId>
 {
     protected override TaskId Create(string value) => new(value);