perf: prebuild the agent image and unify tool installation

[akalipetis]

Stacked on #2, which is stacked on #1 — merge in order. Base is feature-agent-kit, so this PR's diff is only the image work.

Why

Tool installation ran inline in setup.install on every sandbox creation: the Pi installer, two npm installs, and a Webi fetch — roughly 60–90s each time.

The main change: one script, two consumers

Rather than duplicating installs between a Dockerfile and setup.install, tool installation now lives in exactly one place — files/home/.lambda/install-tools.sh:

• the Dockerfile COPYs and runs it, baking the tools into cached layers
• setup.install runs the same script, so the kit still works on a plain shell-docker base or if an image pull fails

Static files under files/home/ land before install commands, which is what makes sharing possible. Every step is command -v guarded and idempotent, so the script no-ops against an image that already has the tools.

setup.install drops from 10 commands to 6, and every survivor needs runtime state an image cannot have:

Command	Why it can't move into the image
ownership chown	volumes mount at runtime
npm proxy config	reads $HTTP_PROXY
install-tools.sh	no-ops on a prebuilt image; the fallback path
Codex config.toml	kept beside the Claude seeding
~/.claude.json	reads $WORKSPACE_DIR
settings.json	paired with the above

Image publishing

.github/workflows/image.yml builds linux/amd64 + linux/arm64 and pushes to ghcr.io/withlogicco/sbx-kit-lambda on pushes to main, on tags, weekly (tool versions float), and on workflow_dispatch. Public, so no registry credential. The workflow asserts the agent-image contract against the pushed digest — agent is UID 1000, all four binaries resolve, and lambda really points at pi.

On sandbox.build

It's declared alongside image so the image's origin is discoverable from the spec, but it buys no behavior. sbx accepts it and warns:

WARN: field "sandbox.build" is accepted but not yet implemented: Dockerfile
      builds are accepted in the schema but not yet built by the runtime;
      the image is taken from sandbox.image

That notice on every validate/inspect is expected, not a regression. Declaring build without image is not an option — the kit fails validation outright:

INVALID: artifact: sandbox.build is accepted in the schema but not yet
         implemented — specify sandbox.image

Worth revisiting if sbx ever implements it: for a kit consumed over git+https://, honouring build would switch every user from pulling a cached multi-arch image to building locally on first create. Pulling from GHCR is the better end state, not a stopgap.

Two things to know before merging

⚠️ Unverified: that sbx prefers a local image over pulling. sandbox.image names a GHCR tag that doesn't exist yet. The intent is that a local docker build -t ghcr.io/withlogicco/sbx-kit-lambda:latest . satisfies it, so nothing changes when CI takes over — and the lambda shell helper builds on demand, with --build to force a rebuild. This could not be tested: the Docker daemon was down. If sbx always pulls, point image back at docker/sandbox-templates:shell-docker until CI publishes; the guarded script makes that a one-line change.

The image has never been built. Same reason. The workflow's contract check is its first real test, so expect to iterate on the Dockerfile if that step fails.

Also note a local build masquerades as the published tag, so docker image inspect is the only way to tell whether you're on a local build or the real one.

🤖 Generated with Claude Code

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-31T15:45:16.817896Z	f3793cd	PR opened

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Reviewed commit: f3793cd768

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