docs(positioning): align docs with agent infrastructure

[drewstone]

Summary

• Lead docs with Tangle's five AI agent infrastructure layers: Sandbox, Runtime, Router, Integrations, and observability through Intelligence.
• Put evaluation inside the layers and products. Replace the old four-layer vision diagram and vocabulary.
• Present hosted assistants as the first packaged use case in preview; keep the first completed reply and request-to-resolution flow unverified.
• Distinguish standard managed provider routing from explicitly selected operator routes. State that on-prem is planned, with no installation path documented today.

This follows the 2026-09-24 company decision and corrects the assistant-first framing introduced in #184.

Verification

• yarn build passed: 289 static routes, 247 Pagefind pages indexed.
• yarn format:check, yarn check:harness-copy, yarn check:release-notes, and yarn check:tnt-core-sync passed.
• Chrome on the local export at 1280px and 400px rendered the new copy and links with no horizontal overflow. The architecture page rendered five layer rows; Router rendered the provider route boundary. All eight linked routes exist in the export.
• No production assistant reply or on-prem availability is claimed.

Screenshots

Current main before this change: desktop, mobile.

After (desktop):

After (mobile):

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[tangletools]

Reviewed exact head 2ded3f4 against main 9e00dfb. The landing, vision architecture/concepts/use cases, AI overview, Router page, and metadata now use the five infrastructure layers from the 2026-09-24 company decision. Evaluation stays inside each layer and product; hosted assistants remain a preview with the first completed text reply and full request-to-resolution flow unverified; on-prem is a plan without an installation claim. Standard provider routing and explicit operator routes are separated. Current-head copy, links, endpoints, format, lint, build, TNT sync, and Netlify preview checks passed. The preview serves the revised landing, architecture, and Router pages with HTTP 200. No open P1 finding.