Add Depone V108 Agent Fabric reference adapter fixture

[moonweave]

Summary

• Adds the V108 deterministic Agent Fabric reference adapter fixture for the local shell harness.
• Introduces build_reference_adapter_fixture(...) and validate_reference_adapter_fixture(...) with A0-claims-only capture semantics.
• Ships a fixture JSON and validates agent-fabric-reference-adapter-fixture through depone validate-contracts.
• Documents the V108 boundary: no live models, no command execution, no productivity/quality/superiority claims.

Safety boundary

• Fixture-only: adapter.executes_commands must be false.
• Capture material remains non-authoritative A0-claims-only.
• Agent self-reported outputs under observer-owned evidence paths are rejected.
• This PR is stacked on PR Implement Depone V107 Agent Fabric contracts #9 and should be reviewed after/with the V107 contract/compiler slice.

Verification

• python3 -m py_compile depone/agent_fabric/reference_adapter.py depone/agent_fabric/__init__.py depone/cli/validate_contracts.py
• python3 tests/test_agent_fabric_reference_adapter.py
• python3 -c 'from depone.agent_fabric.reference_adapter import _self_test; _self_test()'
• python3 -m depone compile --self-test
• python3 -m depone validate-contracts --self-test
• python3 -m depone validate-contracts --all
• git diff --check
• python3 scripts/check_contract.py --tier changed
• python3 scripts/check_contract.py
• strict changed-files secret assignment scan: no matches