feat(tui): CYP tree folding in CYPTreeOverlay

[Flare576]

Summary

In the TUI CYP context overlay, allow the user to fold/collapse subtrees to reduce visual noise in long branching conversations.

Design Notes (from rooms-refinement Tier 4)

• collapsedIds signal tracks which node IDs are folded
• getSubtreeIds helper filters those nodes out of visibleLines
• ~20 lines of logic, no OpenTUI involvement — pure state management
• Keybinding surface TBD: simple (z to toggle) or vim-faithful (za/zM/zR/zo/zc)
• OpenTUI has no native folding concept, so this is entirely self-contained

Scope

TUI only — web CYP tree uses ReactFlow which has its own collapse story.