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
task