feat(tools/gitlab): add tracker + forge bridge for GitLab-hosted projects

[potiuk]

Add tools/gitlab/ as a forge + tracker bridge analogous to tools/github/. GitLab is the dominant non-GitHub forge in our adopter universe.

Capabilities to provide (mirror the GitHub bridge):

• Issue read/write: issue view/list, issue create/edit/close, issue comment
• MR read/write: mr view/list, mr create/edit/merge, mr comment, mr review
• Project metadata: labels, milestones, members
• Project boards (boards / iterations) read/write
• CI: pipelines read for status checks
• Security advisories: GitLab has its own advisory database — useful for the security-* skill family

Why: GitLab hosts Python core infra and several PEP-related repos (gitlab.com/python-org), GNOME (gitlab.gnome.org — all of GNOME), Debian Salsa (salsa.debian.org — all of Debian packaging), KDE (invent.kde.org), Freedesktop (gitlab.freedesktop.org — Mesa, X.org, Wayland, NetworkManager, PipeWire), Inkscape, GIMP, Xfce, Buildroot, the Linux Foundation Energy projects, and the entire self-hosted-enterprise forge market. A project-agnostic framework that can't speak GitLab forecloses a substantial slice of OSS.

Reference:

• GitLab REST API: https://docs.gitlab.com/api/api_resources/
• Existing GitHub bridge to mirror: tools/github/ — the issue-body field-schema pattern in issue-template.md is the convention to follow for GitLab issue templates
• The python-gitlab library is a clean reference impl
• For project-board parity, GitLab's "Boards" + "Iterations" are the analogue of GitHub Projects