fix(cli): emit json for schedule status

[upsetbit]

Summary

Fixes #293.

Makes prosa schedule status --json emit valid JSON instead of TSV text.

What changed

• Added a JSON status payload for schedule status when the global --json flag is present.
• Includes status, installed, and installed-only unit / interval fields.
• Added direct tests for installed and not-installed JSON payloads.

Validation

• go test ./internal/cli -run 'TestEmitScheduleStatusJSON' -count=1
• go test ./internal/cli/... -race
• go build -o ./bin/prosa ./cmd/prosa
• Manual check: prosa schedule status --json | python3 -m json.tool