fix: reject missing nox policy option values cleanly

[doublewhy]

Problem

_split_policy_session_args and _parse_hygiene_posargs index posargs[index + 1] for --base-rev and --requirement-uid. A missing value raises an unhelpful IndexError inside canonical verification instead of a stable configuration diagnostic.

Requirement: GOV-901.

Acceptance

• Both parsers share one guarded value reader.
• Missing values and another option in value position fail with an actionable ValueError naming the option.
• Valid policy/hygiene forwarding behavior remains unchanged.
• Table-driven tests cover both options and both parsers.
• Lint and targeted policy tests pass.