remove old dashboard.ts, superseded by debug-dashboard.ts

The debug-agent's debug-dashboard.ts is a strict superset:
- Has everything dashboard.ts had (health, bridge, sessions, todos, heartbeat)
- Adds activity feed (live tail of control-agent JSONL)
- Adds per-agent uptimes and bridge process uptime
- Better layout (3 rows vs 4, no redundant last-event row)

Nothing references pi/extensions/dashboard.ts anymore.

Author: Baudbot