Fix self-monitoring spawn_agent liveness

[Aaronontheweb]

Summary

• Classify tool liveness as opaque or self-monitoring.
• Keep opaque tools on wall-clock budgets while allowing spawn_agent to rely on child watchdogs after startup.
• Add deterministic watchdog, pipeline, and spawn_agent regression coverage.

Validation

• dotnet test src/Netclaw.Actors.Tests/Netclaw.Actors.Tests.csproj --filter "FullyQualifiedName~StreamingToolCallTests|FullyQualifiedName~SessionToolExecutionPipelineTests|FullyQualifiedName~SpawnAgentStreamingTests"
• dotnet test Netclaw.slnx
• dotnet slopwatch analyze (existing unrelated TUI timing warnings only)
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify

Fixes #1450