feat: add harness failure context

[cafitac]

Summary

• add a top-level failure_context block to harness-style auto/compile reports in CLI, SDK, and MCP
• keep existing final_failure_classification / failure_classification payloads, but expose the operator-facing failure category, retryability, human-handoff signal, summary, failure_reason, and stop_reason in one stable block
• cover success, final-test failure, generate failure, and probe failure cases with regression tests

Testing

• uv run --extra dev pytest tests/unit/sdk/test_client.py tests/unit/mcp/test_tools.py tests/unit/cli/test_auto_command.py tests/unit/cli/test_compile_command.py -q
• bash scripts/check-python.sh
• bash scripts/verify-ai-harness.sh
• uv run --extra dev python - <<'PY' ... failure_context smoke ...