docs(agents): rewrite Agents docs for the new agents service

[sam2tom]

Summary

• Rewrite agents/ guides, Agents/Sessions/Skills API reference and public/openapi.yaml against sandbase-agents on monorepo main (3fb3065fc0).
• New reference pages: Services, Schedules, MCP Connections, Secrets, Agent catalog/restore/unarchive/replace/delete, Session turns/items/files/cancel/reconnect/rename, Skill versions/catalog.
• New agents/openai-compatibility.md: what works with the official OpenAI SDK, what returns 400/404, and SDK examples (Python + Node).
• environment is no longer documented as a request parameter (SandBase is fully hosted); it is only mentioned for SDK compatibility.
• Archive retired content to _archived/: Deployments, Embeds, Endpoints (ACP/MCP), Environments, site-agent guide, use-cases. Remove sessions/list-events.md.
• Update Agents mentions in home page, getting-started, for-agents, FAQ, sidebar.

Testing

• npm run validate passes (OpenAPI lint, API reference/route consistency, links, SEO, build, built public surface).
• Live calls against the TEST environment: 91/91 operations exercised, 88 status codes match the docs; the 3 mismatches are backend/test-env issues, not doc errors (see below). 34 error-path cases, 32 as expected.
• OpenAI SDK: openai-python 3.24.0 40/40, 3.13.0 38/40 (missing SDK methods only), openai-node 7.28.0 examples run as published.
• TEST runs an origin/test build, not main; source/source_id on Sessions verified from code only.

Known backend issues (not blocking docs)

• Session file upload fails on TEST (attachment_unavailable; Core environment_file_write 400).
• Schedule Run cancel returns 409 while pending and does not take effect.
• Several 400s return only a generic invalid request message.
• /v1/vaults returns plain-text 404.
• Session responses still echo environment.type: openai_hosted.

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