[Chore] agent-url dispatch precedence + housing-exchange eval datasets

[Ref34t]

Driver

While running the Housing Exchange analytics-agent POC eval, --agent-url was silently shadowed by the default --provider openai, so every run hit OpenAI instead of the target HTTP agent (surfaced as a phantom "credit balance too low" / 404 model_not_found). The HA eval corpus also needs a committed home.

Scope

• harness: make explicit --agent-url take precedence over the default provider
• add the 357-question housing-exchange corpus (analytics/maltese/refusal/security/ambiguous) under datasets/
• add the Haiku POC baseline runs under runs/

Acceptance Criteria

• --agent-url <url> routes to the HTTP agent regardless of --provider default
• datasets/ holds the 5 labelled sets; python3 -m evalspec.harness --agent-url ... --all runs end to end
• Haiku baseline reports committed