feat(ui): show elapsed time on agent status lines

[xz-dev]

Summary

• add per-current-status elapsed time to running agent status/activity lines
• reset the per-status timer when the visible activity text changes
• share the timer state across the widget, foreground Agent result, and conversation viewer

Examples

Before, the activity line only showed the current status, which could look stuck during long phases:

● Agents
├─ ⠴ worker  Run rpiv act · ↻16 · 9 tool uses · 292.6k token (15%) · 1317.5s
│    ⎿  running command…
├─ ⠴ worker  Run plannotator act · ↻47 · 27 tool uses · 1.4M token (29%) · 1317.5s
│    ⎿  thinking…
└─ ⠴ worker  Run hindsight act · ↻59 · 49 tool uses · 1.9M token (27%) · 1317.5s
     ⎿  reading…

After, each current activity/status line shows how long that visible status has been active:

● Agents
├─ ⠴ worker  Run rpiv act · ↻16 · 9 tool uses · 292.6k token (15%) · 1317.5s
│    ⎿  running command… · 13.5s
├─ ⠴ worker  Run plannotator act · ↻47 · 27 tool uses · 1.4M token (29%) · 1317.5s
│    ⎿  thinking… · 4.2s
└─ ⠴ worker  Run hindsight act · ↻59 · 49 tool uses · 1.9M token (27%) · 1317.5s
     ⎿  reading… · 28.0s

When the visible activity changes, its timer resets:

⎿  thinking… · 13.5s
⎿  running command… · 0.0s
⎿  running command… · 2.5s

The same elapsed suffix is used in the above-editor Agents widget, foreground Agent tool result, and conversation viewer streaming indicator.

Testing

• npm run lint
• npm run typecheck
• npm run test