refactor(cli): remove local Dockerfile image builds

[eviehoward]

Summary

Remove local Dockerfile and local build-context support from openshell sandbox create --from. --from now selects an existing sandbox, community sandbox name, or image reference only; callers build and tag images with the container engine used by their gateway first. Users must now build and tag an image with the container engine used by their gateway, then pass that image reference to --from.

Related Issue

Closes #3098

Changes

• Remove CLI-side Dockerfile detection and image building.
• Remove the no-longer-needed bootstrap image-build implementation and dependencies.
• Return a message with actionable guidance when --from receives an explicit local path.
• Update custom-image E2Es to build test images explicitly through the active container engine.
• Update documentation, examples, agent scripts, and troubleshooting guidance for the image-first workflow.
• Make agent launcher query gateway driver, then build image in matching Docker or Podman store.
• Reject CONTAINER_ENGINE mismatch and unsupported gateway drivers before build.
• Update CLI and gator agent skills for explicit image builds and gateway-selected container engines.

Testing

• mise run pre-commit passes
• mise run e2e passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated - not applicable

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[eviehoward]

Thanks for the review @2000krysztof, I have addressed the requested changes 🙂

[2000krysztof]

All my concerns were addressed LGTM. Nice work @eviehoward :)

[github-actions]

Label test:e2e applied, but pull-request/3214 does not exist yet. A maintainer needs to comment /ok to test 7a81e5a20ee6b22a7e16ac61ecdcf5317c70e4b9 to mirror this PR. Once the mirror exists, re-apply the label or re-run Branch E2E Checks from the Actions tab.

[github-actions]

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[github-actions]

Label test:e2e-kubernetes applied, but pull-request/3214 does not exist yet. A maintainer needs to comment /ok to test 7a81e5a20ee6b22a7e16ac61ecdcf5317c70e4b9 to mirror this PR. Once the mirror exists, re-apply the label or re-run Branch E2E Checks from the Actions tab.

[SDAChess]

/ok-to-test 7a81e5a