[FEATURE] FleetView: pinned sessions should render in a separate section, not just sort to the top

Summary

In the terminal agent view (claude agents / FleetView), pinning a session with Ctrl+T currently just sorts it to the top of the same flat list. Requesting a visually separate "Pinned" section/group instead, so pinned sessions are grouped apart from the regular session list rather than just reordered within it.

Motivation

• Desktop app sidebar and the claude.ai web sidebar both already have a distinct "Pinned" group with its own header, separate from the rest of the session list (see #78679 for the mobile-parity issue describing this existing desktop/web behavior).
• FleetView (terminal) has no equivalent — pinned sessions are only distinguished by list position, which gets harder to scan as the number of pinned + active sessions grows and things shift around.

Request

Add a distinct "Pinned" section (with its own header) at the top of the FleetView session list, separate from the unpinned session list below it — matching the existing desktop/web sidebar pattern. Existing Ctrl+T pin/unpin behavior stays the same; this is purely about visually grouping pinned sessions apart instead of interleaving position within one list.

Environment

Claude Code CLI, terminal agent view (FleetView), macOS.