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/app/createGatewayEventHandler.ts

@@ -2,6 +2,7 @@ import { STREAM_BATCH_MS } from '../config/timing.js'
 import { buildSetupRequiredSections, SETUP_REQUIRED_TITLE } from '../content/setup.js'
 import type { CommandsCatalogResponse, DelegationStatusResponse, GatewayEvent, GatewaySkin } from '../gatewayTypes.js'
 import { rpcErrorMessage } from '../lib/rpc.js'
+import { topLevelSubagents } from '../lib/subagentTree.js'
 import { formatToolCall, stripAnsi } from '../lib/text.js'
 import { fromSkin } from '../theme.js'
 import type { Msg, SubagentProgress } from '../types.js'
@@ -66,20 +67,12 @@ export function createGatewayEventHandler(ctx: GatewayEventHandlerContext): (ev:
         return min === 0 ? s.startedAt : Math.min(min, s.startedAt)
       }, 0)
 
-      // Match buildSubagentTree semantics: an agent is top-level if it has
-      // no parent OR its parent isn't in the snapshot (orphan).  Otherwise
-      // the disk label would fall back to `${N} subagents` for any turn
-      // whose roots got pruned mid-flight.
-      const ids = new Set(subagents.map(s => s.id))
-      const top = subagents.filter(s => !s.parentId || !ids.has(s.parentId)).slice(0, 2)
+      const top = topLevelSubagents(subagents)
+        .map(s => s.goal)
+        .filter(Boolean)
+        .slice(0, 2)
 
-      const label = top.length
-        ? top
-            .map(s => s.goal)
-            .filter(Boolean)
-            .slice(0, 2)
-            .join(' · ')
-        : `${subagents.length} subagents`
+      const label = top.length ? top.join(' · ') : `${subagents.length} subagents`
 
       await rpc('spawn_tree.save', {
         finished_at: Date.now() / 1000,