feat(gateway): persist session goal contracts

[ik-svc-oc]

Summary

• Add SessionGoalContract persistence on session entries
• Add /goal command support for status/new/lock/unlock/clear
• Inject goal contract ahead of mutable session context
• Include stale compaction-summary hardening tests

Test Plan

• uv run pytest tests/gateway/test_session.py::TestSessionGoalContractRoundtrip tests/gateway/test_session.py::TestBuildSessionContextPrompt::test_prompt_includes_goal_contract_before_transport_context tests/gateway/test_session.py::TestBuildSessionContextPrompt::test_prompt_omits_goal_contract_when_absent tests/gateway/test_goal_command.py tests/hermes_cli/test_commands.py -q

Fork-local PR only; no upstream submission.