feat: add delivery contract preflight

[nikolasvoss]

Spec

• Spec: docs/plans/DELIVERY_CONTRACT_PREFLIGHT_TASK_SPEC.md

Plan

• Plan: docs/plans/DELIVERY_CONTRACT_PREFLIGHT_PLAN.md

Behavior Change

• Behavior Change: yes

Acceptance Criteria -> Validation

AC ID	Validation Type	Location / Command
AC-001	regression	tests/test_delivery_contract.py
AC-002	subprocess	tests/test_runtime_requirements.py
AC-003	unit	tests/test_release_readiness.py
AC-004	regression	tests/test_delivery_contract.py

Validation

• Focused regression tests: PASS
• Full CI: pending

Exception Justification

• Exception requested: no