docs: add Runtime as a community harness sandbox provider

[runtimeteam]

Background

HarnessAgent takes any HarnessV1SandboxProvider as its sandbox, but the
docs name only @ai-sdk/sandbox-vercel and @ai-sdk/sandbox-just-bash.
Runtime (Firecracker microVM sandboxes for agents)
now ships a provider in its withruntime package, withruntime/ai-harness.
This PR is from the Runtime team.

Summary

Adds content/providers/05-community-providers/56-runtime.mdx: install, a
HarnessAgent with createRuntimeSandbox(), a turn, the settings, and what
the session implements (getPortEndpoint with private preview tokens,
setPorts, setNetworkPolicy, resumeSession,
HarnessSandboxAuthenticationError). Docs only; no package changes, so no
changeset.

End-to-End Verification

The provider is tested with @ai-sdk/harness 1.0.124's HarnessAgent
driving a scripted HarnessV1 adapter (no model call): create, the bootstrap
recipe, commands, files, previews, network policy, pause/resume and destroy.
Live run, 25 September 2026, against Runtime's production API with real
sandboxes: all 19 checks passed in 12.6 s, and every sandbox was stopped.

Checklist

• All commits are signed (PRs with unsigned commits cannot be merged)
• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

[runtimeteam]

Rebased on current main; it's a single docs page (community providers) and merges cleanly. Could a docs maintainer take a look when there's a moment? Happy to change anything. — Runtime team