Context resolution logging
This release improves visibility into how Piper prepares step context before handing work to a harness.
What's changed
- Added runtime events for context resolution, including start, per-value resolution, completion, cancellation, and failure states.
- Updated CLI output to show context resolution progress and failures more clearly.
- Improved architecture documentation for the runtime flow and harness boundary.
- Added test coverage for context and runtime value resolution behavior.