fix(host-grants): handle YAML timestamps in config hashing - #1105

Summary

Fixes #1100.

PyYAML resolves unquoted ISO-8601 timestamp scalars into Python datetime.datetime and datetime.date objects. These values previously caused host configuration hashing to fail during canonical JSON serialization.

Changes

• Normalize date and datetime values using deterministic ISO-8601 representations.
• Preserve strict serialization errors for unsupported object types.
• Preserve secret-redaction precedence.
• Add focused regression coverage for YAML timestamps, dates, timezone offsets, hashing determinism, and unsupported values.
• Add an end-to-end synthetic Git repository CLI regression test.

Testing

• 9 focused timestamp regression tests passed.
• 76 related tests passed.
• Synthetic CLI regression verified successfully.
• Ruff checks passed.
• Git diff checks passed.