feat: Implement Scheduler Worker Options and Planner Loop

- Added `SchedulerWorkerOptions` class to encapsulate configuration for the scheduler worker.
- Introduced `PlannerBackgroundService` to manage the planner loop, fetching and processing planning runs.
- Created `PlannerExecutionService` to handle the execution logic for planning runs, including impact targeting and run persistence.
- Developed `PlannerExecutionResult` and `PlannerExecutionStatus` to standardize execution outcomes.
- Implemented validation logic within `SchedulerWorkerOptions` to ensure proper configuration.
- Added documentation for the planner loop and impact targeting features.
- Established health check endpoints and authentication mechanisms for the Signals service.
- Created unit tests for the Signals API to ensure proper functionality and response handling.
- Configured options for authority integration and fallback authentication methods.

src/StellaOps.Cli.Tests/Plugins/CliCommandModuleLoaderTests.cs

@@ -22,6 +22,7 @@ public sealed class CliCommandModuleLoaderTests
 
         options.Plugins.BaseDirectory = repoRoot;
         options.Plugins.Directory = "plugins/cli";
+        options.Plugins.ManifestSearchPattern = "manifest.json";
 
         var services = new ServiceCollection()
             .AddSingleton(options)