feat(ui): aggregate the subagent strip when several runs are active

[tt-a1i]

What

The below-editor strip showed one subagent's name while the right side said 3 running — reading as if the other two didn't exist.

Now:

 ⠇ subagents                    3 running · 41s · 3% ctx · ↓ to manage   ← several active
 ⠇ fix-bin-cache                      41s · 3% ctx · ↓ to manage         ← exactly one
 ✓ repo-structure                    1m20s · 3% ctx · ↓ to manage         ← one settled

• A name only appears when it names the only active subagent; several collapse to an aggregate subagents label and the worded counts carry the detail.
• A lone subagent also drops the now-redundant 1 running count — the spinner and the name already say it.

Verification

• bun run check clean; bun run test: 755 node:test + 29 vitest passing
• Tests updated for the single-run tail and a new multi-run aggregation case; strip rendered in all three states to verify layout