Project pi session trees as Explorations

Umwelten will not copy pi's session format as its own canonical Interaction format. Instead, a pi session tree is treated as source material that can be projected into an Umwelten Exploration graph: each meaningful pi branch path becomes a flat Interaction, while pi branch summaries, compactions, labels, and session metadata become Reflections or Exploration metadata. This keeps Interaction model-facing and flat, preserves pi's richer branching semantics, and lets Umwelten operate as the reflection layer across multiple session sources.