feat: add arctl status command

[ExpertVagabond]

Summary

Closes #17 — adds a top-level arctl status command for quick health checks.

What it shows

• Registry connectivity — reachable/unreachable with URL and server version
• Artifact counts — MCP servers, agents, skills, prompts
• Active deployments count
• CLI version and git commit

Usage

$ arctl status
✓ Registry: http://localhost:12121/v0 (v0.3.2)
ℹ CLI: v0.3.2 (abc1234)

  ARTIFACT       COUNT
  MCP Servers    12
  Agents         3
  Skills         5
  Prompts        8
  Deployments    2

$ arctl status --json
{
  "registry": { "url": "http://localhost:12121/v0", "reachable": true, "version": "0.3.2" },
  "cli": { "version": "0.3.2", "git_commit": "abc1234" },
  "artifacts": { "mcp_servers": 12, "agents": 3, "skills": 5, "prompts": 8 },
  "deployments": 2
}

When the registry is unreachable it prints a hint: Start the daemon with: arctl daemon start

Testing

=== RUN   TestStatusCmd_Reachable
--- PASS: TestStatusCmd_Reachable (0.00s)
=== RUN   TestStatusCmd_Unreachable
--- PASS: TestStatusCmd_Unreachable (0.00s)
=== RUN   TestStatusCmd_JSON
--- PASS: TestStatusCmd_JSON (0.00s)

Skips root PersistentPreRunE (API client setup) since it handles its own client creation for best-effort connectivity checks.