Add IAgent.Description for monitoring-tool tooltips

[jeremydmiller]

External monitoring tools (e.g. CritterWatch) want to render a human-readable explanation of what each running agent does on a given node. Today the only metadata available is the agent's URI, which means tooltips collapse to "you have to know what `wolverinedb://default` means."

This PR adds a default-implemented `Description` property on `IAgent`. The default returns `"{scheme} agent: {uri}"` so existing third-party `IAgent` implementations stay source-compatible with no code change. Built-in agent types override the property with text describing what they actually do:

Agent	Description
`DurabilityAgent` (RDBMS)	"Wolverine durability agent for {uri} — recovers persisted inbox/outbox messages, runs scheduled jobs, and emits persistence metrics."
`RavenDbDurabilityAgent`	Same shape, RavenDB-flavoured.
`CosmosDbDurabilityAgent`	Same shape, Cosmos-DB-flavoured.
`ExclusiveListenerAgent`	"Exclusive listener for {endpoint} — only one node at a time holds the listening role for this endpoint, so the cluster avoids competing consumers."
`LeaderPinnedListenerAgent`	"Leader-pinned listener for {endpoint} — runs only on the cluster's elected leader node, so the listening role moves with leader elections."
`StickyPostgresqlQueueListenerAgent`	"Sticky Postgres queue listener — pinned to the per-tenant database '{name}' for queue '{name}'."

Consuming the description

Monitoring tools that want the per-agent description can read it after looking up the `IAgent`:

```csharp
if (runtime.Agents.TryFindActiveAgent(agentUri, out var agent))
{
var description = agent.Description;
// ...emit to telemetry / monitoring envelope...
}
```

No new public API surface beyond the property — `TryFindActiveAgent` already returns the strongly-typed agent.

Tests

Two new xUnit cases in `CoreTests/Runtime/Agents/IAgentDescriptionTests.cs`:

1. `default_description_includes_uri_scheme_and_full_uri` — verifies a bare-bones IAgent that doesn't override Description still produces a non-empty description containing the scheme and URI.
2. `overridden_description_wins_over_default` — verifies an explicit override takes precedence over the default-interface member.

All four persistence projects (RDBMS, RavenDb, CosmosDb, Postgresql) build clean. Existing test suite unaffected.

Motivation / consumer

CritterWatch issue #93 is the immediate consumer — its Service / Nodes tab wants per-agent tooltips so operators can see at a glance what each running agent is doing without having to recognise URI conventions.