[SDK-6f] Extension point: IOutputSink

[ChrisonSimtian]

Expose IOutputSink as a public extension point for structured output events (logging, telemetry, external observability). Maps to surface 6 in RFC #2.

Scope

• Receives structured build / target / tool events.
• Thin wrapper that's easy to register; plugins can also register Serilog sinks directly via builder.Services (per RFC Q1).
• This is the host-side surface that AI-friendly default console output (terse/structured mode) #104 (AI-friendly output) consumes — confirm shape works for that consumer.

Cross-ref

• AI-friendly default console output (terse/structured mode) #104 — AI-friendly default console output (consumer surface).