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:

1. create isolated Box
2. file create + versioned read
3. Shell observes the same workspace
4. workspace observes a Shell-created file
5. exact reattach to the same Box ID
6. cleanup via Box stop

Why Box fits this seam

Mecatl's environment identity is already provider-open and the placement layer owns environment construction/reattachment. Box maps directly onto that contract: opaque Box ID for durable placement identity, remote file operations for Workspace, and remote command execution for the bound CommandRunner.

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]

Superseded by #1604. The replacement is the same Box adapter rebuilt directly on current main, so upstream GitHub Actions can run normally without the stacked #580 history.