feat(adk): add BoxLite microVM sandbox backend

[DorianZheng]

Adds a BoxLite microVM backend for the ADK, implementing filesystem.Backend, filesystem.Shell, and filesystem.StreamingShell — so an agent's whole workspace (file reads, writes, edits, searches, and shell commands) runs inside a hardware-isolated, self-hosted sandbox.

Follows the maintainers' direction in #903 to place this under ADK as a concrete implementation of the unified filesystem.Backend abstraction (this PR previously targeted components/tool/commandline; fully reworked).

Where local executes on the host and agentkit delegates to a remote sandbox service, BoxLite boots a microVM per backend — dedicated guest kernel, no shared host state, no external service — suited to running untrusted, model-generated commands.

How it maps

Every operation goes through in-guest exec so reads, writes, and commands share one filesystem view. Guest scripts take paths as positional parameters ("$1"), so model-supplied paths never need shell-quoting. The local backend is the behavioral reference throughout (error messages, Read's 2000-line default window, Execute's non-zero output format, streaming completion semantics).

Interface method	In-guest realization
LsInfo	ls -1A
Read	sed -n 'start,endp' line window
GrepRaw	rg --json + same JSON parsing as local (requires ripgrep in the guest image; a POSIX command -v probe returns the same friendly install hint otherwise)
GlobInfo	find -mindepth 1 + host-side doublestar matching
Write	base64 chunks sized under the kernel per-arg limit → unbounded file size
Edit	base64 read → local-parity occurrence semantics → chunked write
Execute	sh -c via Box.Exec
ExecuteStreaming	Box.StartExecution + per-line stdout streaming + drain-barrier Wait

The optional MultiModalReader interface is not implemented yet; planned as a follow-up.

Build tag

The BoxLite SDK is CGO with a prebuilt native library (linux/amd64, darwin/arm64), so the implementation is behind the boxlite build tag with a tag-less doc.go — the default build is empty, keeping the workspace CI green without the native library. Consumers run go run github.com/boxlite-ai/boxlite/sdks/go/cmd/setup once, then build with -tags boxlite. If you'd prefer a dedicated CI job that runs setup and tests -tags boxlite, happy to add it.

Testing

• Default (tag-less): go build ./... / go vet ./... green, zero BoxLite packages in the dependency graph.
• With -tags boxlite on darwin/arm64 (BoxLite SDK v0.9.7), against real booted microVMs (BOXLITE_E2E=1, python:slim):
  • write → windowed read → edit → ls → glob round-trip (special characters included)
  • 300 KiB chunked write, byte count verified in-guest
  • Execute zero and non-zero exits (combined report format)
  • ExecuteStreaming line-by-line streaming
  • GrepRaw verified both ways: against a box with ripgrep installed (match + case-insensitive + glob filter), and the install-hint error on an image without it