Add simple check command

[AzurLiu]

Summary

• add a minimal falsiflow check --config --evidence entry point that prints claim_ready or claim_blocked with reasons
• add a tiny numeric-claim example covering placeholder, weak-lift, and ready evidence
• update README first-run docs, CLI reference, release gates, and regression coverage

Verification

• python3 -m py_compile falsiflow/cli.py falsiflow/release.py scripts/falsiflow_tests/regress_falsiflow_core.py
• python3 scripts/falsiflow_tests/regress_falsiflow_core.py
• make test
• python3 scripts/falsiflow.py release-check --out-dir /tmp/falsiflow_release_check_simple --force --json