Add GitLab issue support

[tupe12334]

• Add IssueRef::GitLab { owner: String, repo: String, number: u64 } variant
• Parse https://gitlab.com/<owner>/<repo>/-/issues/<N> URLs in src/issue/parse/mod.rs
• Add parse_gitlab_remote_url() in a new src/issue/parse/gitlab.rs (mirroring gh.rs)
• Update clone_url() in src/issue/impls.rs to return https://gitlab.com/{owner}/{repo}.git
• Update workspace_dir_name() / branch_name() to return issue-<N> (same as GitHub)
• Handle worktree://open?gitlab_host=gitlab.com&owner=X&repo=Y&issue=N in src/issue/parse/worktree_url.rs
• Remove or update test assertion that explicitly rejects GitLab URLs

[github-actions]

A workspace is ready for this issue.

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