Add Tasks verification planning contract

[sneakocom]

Summary

• add a structured task request and plan wrapper around AV plan v2
• accept exact unchanged trees and repositories with no automated checks without inventing evidence
• expose the contract through task-plan while keeping AV command-free

Safety

• unknown impact and incomplete evidence remain fail-closed
• Tasks retains execution authority and validates all returned commands against an immutable base manifest
• the existing research claim ceiling is preserved

Verification

• npm run verify
• 112 tests passed
• 15 conformance scenarios passed
• determinism checks passed
• AV-EXP-003 frozen verifier passed