Add agent command execution (terminal panel)

- DB: new agent_commands table (id, agent_id, command, status, output, exit_code, timestamps)
- Server: 4 new routes
    POST /api/agents/:id/commands        queue a command (dashboard → agent)
    GET  /api/agents/:id/commands        history (last 50) for dashboard
    GET  /api/agents/commands/pending    agent dequeues + marks running (Bearer auth)
    POST /api/agents/commands/:id/result agent posts output/exit_code (Bearer auth)
  Output capped at 128 KB; Bearer token enforced on agent-facing routes
- Agents (Linux/macOS/Windows): each polls /commands/pending each cycle,
  executes via bash/PowerShell with 60s timeout, posts result back
- UI: ">_" button on each AgentCard opens CommandTerminal panel
    Input line, Run button, history list (status icon + command + time)
    Click any entry with output to expand/collapse its output
    Auto-refreshes every 4 s while open

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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