feat(retrieval): switchable agent_explore Cursor model

[EricNGOntos]

Summary

• Retrieval query can pass agent_explore_model so Cursor explore uses that model for this request; omit it and the current server default stays.
• Cache key includes the model so two models do not share one result.
• Also lands the tool-call fixes: pattern/patterns, loop-contract dependency rule, and node_filter argument wording.

Test plan

• make check
• focused tests: harness resolve, grep, cache digest
• After merge/deploy: POST /api/v1/retrieval/query with use_agentic=true and agent_explore_model set; confirm decision_trace / model in the episode uses the requested name

Made with Cursor