feat(cli): add JSON status output

[AnayGarodia]

Summary

• add agent-handover status --json for CI and dashboard consumers
• emit one stable JSON object containing status, steps, pending work, and checkpoint timestamps
• preserve the existing human-readable output as the default
• document the machine-readable contract and cover it with json.loads

Verification

• python -m pip install -e .
• python -m pytest -q (55 passed)

Closes #5

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