Add GitLab Issues as a backlog manager option

[rcfrias]

Summary

• Adds a new gitlab-issues backlog manager that drives the glab CLI: sandcastle init offers it alongside GitHub Issues and Beads, the sandbox Dockerfile/Containerfile installs glab, and the scaffold writes GITLAB_TOKEN (with a commented GITLAB_HOST for self-hosted instances) into .env.example.
• glab issue close does not accept a comment, so CLOSE_TASK_COMMAND widens from string to readonly string[] and renders as a &&-joined shell pipeline (glab issue note <ID> -m "..." && glab issue close <ID>). The GitHub and Beads entries become single-element arrays.
• New --backlog-manager <name> flag on sandcastle init skips the interactive prompt (parallels the existing --agent / --template flags).

Design notes

• JSON shape parity. GitHub's LIST_TASKS_COMMAND produces [{number, title, body, labels, comments}]. To keep the templates provider-agnostic, GitLab's command reshapes glab issue list -O json via jq into the same shape (number: .iid, body: .description). comments is rendered as []; agents fetch full comments via VIEW_TASK_COMMAND (glab issue view <ID> --comments).
• Arch-aware install. The GitLab CLI install resolves the latest release at build time and downloads the matching .deb for the container architecture (amd64 / arm64), so the same Dockerfile works on both Apple Silicon and Linux hosts.
• Label flow. cli.ts gains a glab label create branch alongside the existing gh label create one, gated on selectedBacklogManager.name. The --label Sandcastle stripping in rewritePromptFiles already handles the long-form flag, so it works for GitLab too without changes.

Test plan

• npx vitest run src/InitService.test.ts — 161/161 pass, including 11 new tests covering registry shape, scaffold output across all 5 templates, Dockerfile install, and .env.example shape for GitLab.
• src/cli.test.ts — added 2 tests for --backlog-manager flag (--help exposure + unknown-value error).
• prettier --check clean on changed files; lint-staged ran on commit.
• tsgo --noEmit — no new errors; only the pre-existing @daytona/sdk errors remain.
• Manual sandbox build with gitlab-issues selected (not run in this PR — the CI image build will exercise it).

Changeset

.changeset/add-gitlab-backlog-manager.md (patch).

[vercel]

@rcfrias is attempting to deploy a commit to the Matt Pocock's projects Team on Vercel.

A member of the Team first needs to authorize it.