docs: make Agent endpoints discoverable

[DanielleWashington]

Summary

• make the Agent create, status, and cancel references visible and indexable in all six locales
• expose the Agent endpoint group in API Reference navigation and link the complete lifecycle from each Agent guide
• align Agent guides, model guidance, CLI docs, snippets, and checked-in OpenAPI with the product default of spark-1-pro
• preserve existing public routes and the former Spark 1 Mini heading anchor

Why

The Agent reference pages existed, but their navigation groups and page metadata kept them hidden from normal discovery. The visible Agent guide and v2 introduction could link into those hidden pages. The docs also described spark-1-mini as the default even though the product request schema defaults to spark-1-pro.

Verification

• rebased cleanly onto current origin/main
• git diff --check passed
• docs.json and all six v2 OpenAPI files parse successfully
• all six Agent navigation groups resolve as visible
• all six OpenAPI files declare spark-1-pro as the Agent default
• all 48 changed rendered routes return HTTP 200 in the local Mintlify preview
• rendered Agent API navigation and absence of robots: noindex verified across all six locales
• product source confirms POST /v2/agent, GET /v2/agent/:jobId, DELETE /v2/agent/:jobId, and the spark-1-pro default

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}