Add native RLM recursive agent design

[ajroetker]

Summary

• add zig/RLM.md with the native Recursive Language Model design, references, placement, query-builder role, and non-goals
• add a shared recursive agent substrate plus recursive execution mode wiring for retrieval and query builder requests
• add query-builder recursive candidate planning with typed join/aggregation artifacts, runtime preflight/planner signals, aggregate catalog stats, and right-key join stats
• update OpenAPI metadata and generated Zig clients for recursive execution, trace artifacts, and incomplete reasons

Tests

• zig build root-test --test-timeout 2m
• git diff --check