Sub-session visibility lost after v2.1.30 session architecture change

Previously, when a session went through multiple automatic context compactions, the /resume list showed these as sub-sessions (segments) under the main session entry. This was very useful for understanding session progression and jumping back to specific points in a long-running session.

Since approximately v2.1.30 (stat-based session loading), all sessions appear as a single flat entry regardless of how many compaction events occurred. While I understand this was a performance optimization (68% memory reduction), the loss of sub-session visibility is a significant UX regression for power users with long-running sessions.

Request: Restore sub-session/segment display in the /resume picker, or provide an alternative way to see compaction history within a session (e.g. expandable entries, or a --verbose flag for /resume).