feat(e2e/go): E2E tests for the Go SDK

User Story

As a contributor working on OpenShell Go SDK I want mise run e2e:go to run E2E tests against a real
gateway. Regressions in sandbox lifecycle, exec, provider credential injection, and workspace management will be caught before they reach SDK consumers.

Problem Statement

The Go SDK (sdk/go/) is tested only by unit tests and sdk/go/openshell/v1/integration_test.go, which is only a very basic smoke E2E test. New E2E Go SDK test suite should be able to catch issues with:

• protocol schema mismatches between the Go SDK and the actual gateway build
• sandbox lifecycle can be managed by the Go SDK
• provider credentials management using the Go SDK
• workspace management

Impact / Why This Matters

The Go SDK (sdk/go/) is tested mostly by unit tests. These tests mock the transport and never talk to a real openshell-gateway. Which means that currently:

• A gateway change can silently break every Go SDK consumer
• Contributors have no mise run entry point to test Go SDK changes end-to-end before opening PR

Proposed Design

• Separate module: E2E tests live in e2e/go/
• No SDK changes: sdk/go/ is untouched

Possible design can be found in draft PR: https://github.com/jiripetrlik/OpenShell/pull/1

Acceptance Criteria

• e2e/go/ is a separate Go module
• All e2e files has //go:build e2e
• Sandbox tests cover CRUD
• Provider tests assert placeholders (not raw secrets)
• Workspace tests cover CRUD
• AGENTS.md documents the e2e/go/ conventions
• mise run pre-commit passes; commits are DCO-signed and follow Conventional Commits

Alternatives Considered

• Rely on unit tests - can be faster to test, but many possible bugs can be missed
• Put the e2e tests inside `sdk/go/ - put E2E tests to a separated directory is much more clear

Agent Investigation

No response

Checklist

• I've reviewed existing issues and the architecture docs
• This is a design proposal, not a "please build this" request

Start by reviewing the existing sdk/go/openshell/v1/integration_test.go and the draft PR linked in the issue, then inspect the proposed e2e/go module conventions. The work is done when build-tagged E2E tests cover sandbox and workspace CRUD, provider placeholder assertions, and the documented mise commands pass alongside mise run pre-commit.