Add `agent-handover status --json` machine-readable output

[hikari716]

Context

agent-handover status prints a human-readable checklist
(src/agent_handover/cli.py). Automation (CI, dashboards) needs a stable
machine-readable form.

Task

Add a --json flag to status that prints the checkpoint state as one JSON
object to stdout, e.g. {"status": "in_progress", "steps": {...}, "pending": [...]}.
Human output stays the default when --json is absent.

Acceptance criteria

• agent-handover status --json prints valid JSON (one object, no extra logs)
• Exit code unchanged (0)
• A test asserts the shape via json.loads on captured stdout
• README documents the flag

Pointers

• src/agent_handover/cli.py (status branch)
• Checkpoint.load() / Checkpoint.pending_steps() in src/agent_handover/checkpoint.py
• Tests: tests/test_cli.py