refactor(tui): consolidate agents overlay — share duration/root helpers via lib

Pull duplicated rules into ui-tui/src/lib/subagentTree so the live overlay,
disk snapshot label, and diff pane all speak one dialect:

- export fmtDuration(seconds) — was a private helper in subagentTree;
  agentsOverlay's local secLabel/fmtDur/fmtElapsedLabel now wrap the same
  core (with UI-only empty-string policy).
- export topLevelSubagents(items) — matches buildSubagentTree's orphan
  semantics (no parent OR parent not in snapshot). Replaces three hand-
  rolled copies across createGatewayEventHandler (disk label), agentsOverlay
  DiffPane, and prior inline filters.

Also collapse agentsOverlay boilerplate:
- replace IIFE title + inner `delta` helper with straight expressions;
- introduce module-level diffMetricLine for replay-diff rows;
- tighten OverlayScrollbar (single thumbColor expression, vBar/thumbBody).

Adds unit coverage for the new exports (fmtDuration + topLevelSubagents).
No behaviour change; 221 tests pass.

ui-tui/src/__tests__/subagentTree.test.ts

@@ -5,11 +5,13 @@ import {
   descendantIds,
   flattenTree,
   fmtCost,
+  fmtDuration,
   fmtTokens,
   formatSummary,
   hotnessBucket,
   peakHotness,
   sparkline,
+  topLevelSubagents,
   treeTotals,
   widthByDepth
 } from '../lib/subagentTree.js'
@@ -367,3 +369,42 @@ describe('formatSummary', () => {
     ).toBe('d3 · 7 agents · 124 tools · 2m 14s · ⚡2')
   })
 })
+
+describe('fmtDuration', () => {
+  it('formats under a minute as plain seconds', () => {
+    expect(fmtDuration(0)).toBe('0s')
+    expect(fmtDuration(42)).toBe('42s')
+    expect(fmtDuration(59.4)).toBe('59s')
+  })
+
+  it('formats whole minutes without trailing seconds', () => {
+    expect(fmtDuration(60)).toBe('1m')
+    expect(fmtDuration(180)).toBe('3m')
+  })
+
+  it('mixes minutes and seconds', () => {
+    expect(fmtDuration(134)).toBe('2m 14s')
+    expect(fmtDuration(605)).toBe('10m 5s')
+  })
+})
+
+describe('topLevelSubagents', () => {
+  it('returns items with no parent', () => {
+    const items = [makeItem({ id: 'a', index: 0 }), makeItem({ id: 'b', index: 1 })]
+    expect(topLevelSubagents(items).map(s => s.id)).toEqual(['a', 'b'])
+  })
+
+  it('excludes children whose parent is present', () => {
+    const items = [
+      makeItem({ id: 'p', index: 0 }),
+      makeItem({ depth: 1, id: 'c', index: 0, parentId: 'p' })
+    ]
+
+    expect(topLevelSubagents(items).map(s => s.id)).toEqual(['p'])
+  })
+
+  it('promotes orphans whose parent is missing', () => {
+    const items = [makeItem({ id: 'a', index: 0 }), makeItem({ depth: 1, id: 'orphan', index: 1, parentId: 'ghost' })]
+    expect(topLevelSubagents(items).map(s => s.id)).toEqual(['a', 'orphan'])
+  })
+})