[Dashboard] B1 implementation: first-party BigQuery data agent shared to Data Studio (natural-language Q&A over agent_events)

Implementation issue for the **B1 option selected on #402**, per its closure contract. The decision evidence and criteria matrix live in [`docs/conversational-analytics-decision.md`](https://github.com/GoogleCloudPlatform/BigQuery-Agent-Analytics-SDK/blob/main/dashboard/looker_studio/docs/conversational-analytics-decision.md) (evaluation record completed 2026-08-11, merged in #416).

## Scope — everything the handoff must own (from the decision doc)

- [ ] **Versioned agent-definition + golden-question artifact** preserving the curated-view, timestamp-predicate, cost (`big_query_max_billed_bytes`), and #382 sharp-edge contracts (prompt+completion = `prompt_token_count + candidates_token_count`, thinking separate; zero-latency local tool events are valid). Built once, shared with #396 (host-neutral core vs per-host adapters, per the doc).
- [ ] **Provisioning guide or command**: curated allowlist view (raw content, prompts, user IDs, error text, authorization data, trace identifiers excluded), **least-privilege IAM as the enforcement boundary** (viewers get BigQuery access to the view only; agent IAM is not a data boundary), agent creation + publish/share to Data Studio, and an **approved regional endpoint** (`global` has no residency commitment).
- [ ] **Report and configurator link-outs** that open Data Studio in a new tab, make the **non-transferred date/agent filters explicit** for re-entry, and set the **answer-latency expectation (observed 14.4–33.3 s — directional, not an SLO)** — the same expectation-setting discipline as the configurator's provisioning wait note.
- [ ] **Compatibility tests + named owner** tying the artifact version to the `events_v1` schema and Data Studio's Preview surface (Preview-status changes must fail loudly, not silently).
- [ ] **Fresh production asset**: the evaluation DataAgent `bqaa-issue-402-eval-20260810` is delete-don't-promote (#404 teardown checklist); the production agent is provisioned new from the versioned artifact.

## Ownership

Proposed owner: **@haiyuan-eng-google** (canonical-template and test-infrastructure holder), pending explicit acceptance on this issue — per #402's closure contract, that acceptance (or a replacement owner) is what allows #402 to close as transferred.

## Related

- #402 — decision issue (closes on this issue's ownership acceptance or on ship, per its closure contract).
- #416 — the evaluation record (merged).
- #396 — shared schema/golden-question artifact consumer.
- #404 — tracking; teardown checklist covers the evaluation-era assets.