Default the checkpoint path from $AGENT_HANDOVER_CHECKPOINT

[hikari716]

Context

The CLI defaults --checkpoint to .agent-handover/checkpoint.json
(src/agent_handover/cli.py). Users running one repo from multiple agents or
worktrees want to set this once via the environment.

Task

When --checkpoint is not given, fall back to the AGENT_HANDOVER_CHECKPOINT
environment variable, then the current default.
Precedence: --checkpoint > $AGENT_HANDOVER_CHECKPOINT > default.

Acceptance criteria

• Flag overrides the env var (test)
• Env var used when flag absent (test)
• Falls back to default when neither is set (test)
• README documents the env var

Pointers

• src/agent_handover/cli.py (argparse setup), tests/test_cli.py