feat: sidebar hierarchy view

[aterrylu]

Summary

• Adds a toggleable hierarchy view to the Agents sidebar that organizes agents by their org chart parent-child relationships
• Parents show a blue left border bar that acts as expand/collapse toggle (click bar to toggle, click row to switch pane)
• Drag-to-reorder within sibling groups, persisted to localStorage
• Stopped agents filtered from hierarchy when eye toggle is off (preserves parents with live descendants)

Changes

• Sidebar.tsx: Extracted SessionRow/ExitedRow components; added HierarchyNodeRow with tree rendering, useOrgChartData hook, mergeOrgWithSessions tree builder with prune/order logic
• store.ts: Added sidebarViewMode ("flat"/"hierarchy") and hierarchyOrder (per-group ordering) to persisted state
• Codicon.tsx: Added chevron-down, chevron-right, list-flat, list-tree icons

Design decisions

• Hierarchy data comes from GET /api/org (pre-built tree) merged with live sessions, not from the manager field on sessions (which isn't reliably populated)
• No chevron icons for expand/collapse — uses colored left border bar + click-to-toggle on the bar zone
• Drag reorder is scoped to sibling groups (can't drag across hierarchy levels)

Test plan

• Toggle between flat and hierarchy view using the icon next to "AGENTS"
• Verify parent agents show blue left border, children are indented
• Click the blue bar area to collapse/expand children
• Click an agent row to switch to its terminal (should NOT toggle children)
• Toggle "show stopped" eye icon — hierarchy should filter stopped agents
• Drag agents within a group to reorder — verify order persists on reload
• Switch back to flat view — verify original behavior unchanged

🤖 Generated with Claude Code

[nox-0x]

/tmp/pr120-comment.txt

[nox-0x]

Clean, well-scoped feature addition. Tree-based hierarchy view with drag-to-reorder, collapse/expand, and flat/hierarchy toggle is solid. Extracting SessionRow/ExitedRow/HierarchyNodeRow components reduces duplication and improves readability. Polling-based org chart merge is appropriate. Left one non-blocking warning about using node.org.name as React keys (potential duplicate-name collision) — worth addressing before shipping but not a blocker.