SignalCli receive liveness watchdog, OTel exporter auth, and messaging reliability fixes

[f2calv]

Summary

Hardens the Signal communications pipeline with a passive receive-liveness watchdog,
adds authentication support to the OpenTelemetry exporter, improves the SignalCli
integration test suite, and corrects the Signal reaction feedback so failures are no
longer reported as successes. Also refreshes NuGet dependencies and adds design
documentation.

Changes

SignalCli receive liveness

• Added a passive receive-staleness watchdog to SignalCliJsonRpcClientService:
  tracks the time since the last inbound WebSocket frame and surfaces a configurable
  staleness timeout (ReceiveStalenessTimeoutMs) so a silently dead receive stream is
  detected rather than hanging indefinitely.
• New _SignalCliConfig options to drive the watchdog behaviour.

Messaging reliability

• Fixed the Signal reaction feedback in CommunicationsBgService: the green tick (✅)
  was previously sent unconditionally, even when agent inference failed and produced no
  response. The reaction now reflects the actual outcome — ✅ only on a successful,
  non-empty response, and ❌ (red cross) on failure or empty result.

Observability

• Added authentication support to the OpenTelemetry (OTLP) exporter so traces/metrics
  can be exported to a secured collector endpoint.

Testing

• Added xUnit traits (SignalCliTraits) for categorising SignalCli integration tests.
• Improved integration-test teardown for deterministic cleanup of server-side state.
• Additional SignalCli REST client integration test coverage and TestBase tweaks.

Dependencies

• Routine NuGet package updates (Directory.Packages.props).

Documentation

• New design docs: 002-signalcli-receive-heartbeat.md (receive-liveness investigation
  and the passive watchdog decision) and 003-speckit-adoption.md.

Notes

• The active receive-heartbeat approach explored in design doc 002 was found to be
  unworkable for a linked signal-cli device and is documented as parked; the committed
  solution is the passive watchdog only.