feat: add Ascii Box remote execution environment

[zozo123]

Summary

Adds an Ascii Box-backed Mecatl execution-environment / placement-provider adapter directly on main.

What it provides

• server.PlacementProvider + exact PlacementReattacher backed by the Box API
• durable EnvironmentRef{Kind: "box", ID: <box-id>, Revision: "box-api-v1"}
• Box filesystem adapter implementing Mecatl's versioned Workspace contract
• Shell CommandRunner bound to the same Box workspace namespace
• POSIX-like namespace operations (ReadDir, Remove, Rename, CopyFile)
• create / readiness / stop / resume lifecycle handling
• noEnv: true by default so account/model/GitHub/SSH secrets are not injected into agent sandboxes
• bounded API responses, path confinement, safe shell quoting, HTTPS-only API endpoint configuration
• offline httptest coverage for creation, exact reattachment, version conflicts, grep, path escape, auth/error handling
• opt-in real API smoke test behind box_live

The API key is host-side configuration only and is never placed in EnvironmentRef, the Box environment, tests, or repository state.

Live verification

The live contract test is opt-in and excluded from normal CI:

BOX_API_KEY=... go test -tags=box_live ./internal/adapter/boxenv -run TestLiveBoxEnvironment -v

It exercises create, file read/write, Shell/workspace namespace agreement, exact reattachment, and cleanup.

Scope

This PR intentionally keeps Box isolated in internal/adapter/boxenv; it does not add a parallel sandbox abstraction or leak Box concepts into engine/. Operator-facing execution-placement selection can be wired separately once the provider-registration surface is finalized.

[zozo123]

CI note: all pull-request workflows (CI, Deslop, e2e-live, perf) are currently action_required before any jobs are created. This is GitHub's fork-contributor workflow-approval gate, not a test failure. A Stacklok maintainer needs to click Approve and run workflows on this PR; no repository secrets are requested by the PR CI path.

[zozo123]

One more status note: the remaining red Vercel check is also a permission gate (Authorization required to deploy for the Stacklok Vercel team), not a build/deploy error from this change. So the PR currently needs maintainer approval for GitHub Actions and Vercel before any real CI result can go green/red.

[zozo123]

E2E evidence (tested with gh)

Tested PR revision 0dab52aa2e2b67c5baa18c75dd2b7372094c9152 locally on macOS arm64:

• go test ./internal/adapter/boxenv ✅
• go test -tags=box_live ./internal/adapter/boxenv -run TestLiveBoxEnvironment -count=1 -v ❌
  • Create flow reaches the live Box, but CreateFile fails because the file-read probe receives HTTP 400 box_direct_failed: ENOENT for a missing file. The adapter currently only normalizes HTTP 404 to fs.ErrNotExist.

I reproduced a minimal local fix that normalizes the Box API's ENOENT response to fs.ErrNotExist; with that fix applied, the same live contract test passes in 5.4s and exercises create, file read/write, Shell/workspace namespace agreement, exact reattachment, and cleanup. The fix is not pushed because this checkout does not have permission to push to stacklok/mecatl (GitHub returned HTTP 403).

[zozo123]

Follow-up: local compatibility fix verified

The live failure is reproducible and isolated to missing-file semantics: Box returns HTTP 400 with box_direct_failed / ENOENT, not HTTP 404. I implemented and verified a local fix that treats both forms as fs.ErrNotExist (including the CopyFile probe path).

Verification with the fix applied:

go test ./internal/adapter/boxenv
ok   github.com/stacklok/mecatl/internal/adapter/boxenv

go test -tags=box_live ./internal/adapter/boxenv -run TestLiveBoxEnvironment -count=1 -v
--- PASS: TestLiveBoxEnvironment (4.35s)
PASS

The fix is committed locally as f57633b1d; pushing is blocked because the authenticated GitHub account has no write permission to stacklok/mecatl (HTTP 403).

[zozo123]

Pushed the verified fix to the PR head branch (f57633b1d). The live Box contract test now passes end-to-end, including create, file read/write, Shell/workspace namespace agreement, exact reattachment, and cleanup. The remaining Vercel check is an external authorization-required deployment failure, unrelated to the Go tests.

[jhrozek]

hey @zozo123 you opened 3 PRs with essentially the same feature but they don't appear to be the same, which one of them is the current one?