Plan 5d: Runners fleet view

[theaboutbox]

Summary

Plan 5d. Replaces the runners route stub with a fleet view that aggregates four existing endpoints. No new backend.

• KPI strip: current Runner mode (with badge + source), Live Fly sessions, Capacity used / max (sum of caps), Reaper 24h count + last sweep ago.
• Override banner appears when runner-mode !== 'default' explaining all projects route through the override regardless of their per-project mode.
• Live Fly sessions table (top 8) — read-only here; manage on Sessions page.
• Per-project execution mode table with mode badge, effective mode (matches per-project unless override active), and a colored capacity-utilization meter (green / amber ≥75% / red ≥100%).
• Tolerates /api/sessions 503 (Fly app not configured) and /api/reaper/summary failure as best-effort — neither shows a page-level error.
• 6 structural tests; 679 tests pass.

Plan: docs/superpowers/plans/2026-04-30-admin-runners.md. PR base: admin-overhaul.

Test plan

• #runners shows mode + sessions + capacity + reaper KPIs
• Switching runner mode on the Reaper page (then refreshing here) updates the badge and toggles the override banner
• When Fly sessions app is unset, sessions count shows 0 and the table is empty (no error)
• Per-project meter turns amber at 75% and red at 100%

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