docs(sandbox): clarify harness routing boundaries

[drewstone]

Summary

• Add a canonical Sandbox Harnesses page covering SDK backends, Sandbox UI picker exposure, and AI Agent Sandbox blueprint sidecar capability discovery.
• Reframe OpenCode as one coding harness inside the sandbox, not the product boundary.
• State the routing model plainly: every coding harness runs in the sandbox; model calls use Tangle Router unless profile, user, or operator settings point elsewhere.
• Add a CI copy guard so future OpenCode mentions must include peer harness context or link to the harness page.

Verification

• yarn check:harness-copy passed
• node /home/drew/code/dotfiles/claude/skills/docs-slop-audit/scripts/scan-docs-slop.mjs --json pages/ai pages/vibe pages/infrastructure pages/blueprints components/LandingPage.tsx returned 0 findings across 29 files
• yarn format:check passed
• yarn check:tnt-core-sync passed
• git diff --check passed
• yarn build passed: Next built 262 static pages, next-sitemap generated 1 sitemap index + 1 sitemap, Pagefind indexed 230 pages

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[tangletools]

✅ Auto-approved PR — e66101f8

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_{tangletools · auto-approval · reason: blanket_auto_approve · 2026-06-15T11:16:59Z}