Integrate Orbit tools into Planner Agent
## Summary Wire Orbit (GitLab Knowledge Graph) tools into the Planner Agent so it can leverage graph-style queries for richer planning context. ## Requirements * [x] Wire Orbit tools (`get_graph_schema`, `query_graph`) into Planner via DWS (MCP now, native tools later) * [x] Update system prompt to prefer Orbit for graph-style questions * [x] Update system prompt to fall back to existing tools when Orbit is empty or out-of-scope * [ ] Feature flag + rollout plan defined
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