feat(orchestration): live activity line in Mission Control Spawned Work rows

[zaridan]

What

Adds a live activity line beneath each worker name in the Orchestrator Mission Control "Spawned Work" rows, so a worker row shows what its agent is currently doing — not just a status dot.

Why

Mirrors the activity line DashboardAgentRow already renders elsewhere in the app. Previously a spawned-work row showed only the agent state dot + worker name, giving no at-a-glance signal of the agent's current activity.

How

• Extracted selectFreshestWorktreeAgentEntry from deriveWorktreeAgentDotState in worktree-agent-dot-state.ts so the dot and the activity line derive from one rule (the freshest agent-status entry among a worktree's tabs), instead of duplicated logic that could drift.
• Added selectWorktreeAgentActivityText (worktree-agent-activity-text.ts): returns the freshest agent's orchestration label / prompt via getAgentRowPrimaryText, falls back to the state label when the agent is live but reported no prompt, and returns null when no agent has reported (so the row omits the line rather than showing a placeholder).
• OrchestratorMissionControl.tsx renders the worker name + truncated activity line (with title tooltip) in the row header.

Tests

New OrchestratorMissionControl.spawned-activity.test.tsx covers: prompt text rendered under the worker name, state-label fallback on empty prompt, and the line omitted when no agent has reported.

Typecheck, full pnpm test (20498 passed), oxlint on changed files, and the electron-vite build all pass.

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