Implement scripts/knowledge_gate.py and fail CI on KB benchmark regressions #32

Summary

Onboarding already has scripts/eval_gate.py in CI. Knowledge-base benchmarks are documented with thresholds but the gate script is still marked future.

Context

• docs/benchmarks/knowledge-base.md (citation p95 < 5 ms, search p95 < 100 ms, zero org-isolation failures)
• scripts/eval_gate.py + existing eval-gate CI job (pattern to copy)
• scripts/benchmark_knowledge_base.py

Acceptance criteria

• python scripts/knowledge_gate.py exits 0 on current mock benchmarks
• Parses JSON from the knowledge-base benchmark
• CI job fails when thresholds regress (not continue-on-error forever)
• Unit test for threshold parsing
• Docs updated so “future” is removed

Distinct from #25 (nightly live-provider smoke).