kernel observer: root-level observability with OTel-aligned levels

[JaimeStill]

Summary

Promotes orchestrate/observability/ to a root-level observability/ package with OTel-compatible severity levels, integrates the Observer pattern into the kernel runtime loop replacing the ad-hoc *slog.Logger, and migrates all orchestrate imports.

Closes #25

Changes

• New observability/ package — Foundation-level (Level 0) with Observer interface, Event struct, OTel-aligned Level type (values ARE OTel SeverityNumbers), SlogObserver, NoOpObserver, MultiObserver, and global registry
• Kernel observer integration — Replace *slog.Logger with observability.Observer, add WithObserver option, 7 kernel event types, default SlogObserver(slog.Default())
• Orchestrate migration — All imports updated from orchestrate/observability to observability, Level field added to all event emissions, new node.state event separates state snapshots from node.complete
• Decentralized event types — Each package defines its own event type constants (orchestrate/state/events.go, orchestrate/workflows/events.go, kernel/observer.go)
• 100% test coverage on new observability package, all existing tests updated and passing