Type Forge incident ref envelopes

[im-sham]

Summary

• add a typed IncidentRefEnvelope boundary model for Forge Proofhouse IncidentRef output
• keep Incident.to_ref_envelope() backward-compatible by returning the same dictionary shape
• route CLI and MCP IncidentRef JSON serialization through the typed envelope model

Verification

• RED: /tmp/ops-g375-venv/bin/python -m pytest tests/test_models.py -k typed_boundary
• /tmp/forge-g376-venv/bin/python -m pytest tests/test_models.py tests/test_cli.py::test_ref_command_prints_incident_ref tests/test_mcp_http.py::test_forge_schema_exposes_centralized_structured_axis_metadata
• /tmp/forge-g376-venv/bin/python -m ruff check forge_cli tests
• /tmp/forge-g376-venv/bin/python -m pytest
• git diff --check