feat(conformance): AgentPassport good/bad fixtures + validator patch [T5-1..T5-5]

[mdheller]

Add AgentPassport conformance fixtures + agent-class validation to tools/validate_contract.py, consuming the merged AgentPassport model (sourceos-spec T0-1). good/agent-passport-classified (valid third_party) passes; bad/agent-passport-unclassified (no class → 'unclassified') and bad/agent-passport-self-elevated (third_party+suppress+system_bundle → 'class elevation') are rejected. + semantic/agent-system.jsonld import (T0-3).

Source: Tranche Work Orders T5-1..T5-5.