docs(skills): add contributing section to bundled glab skill

What does this MR do?

Adds a Contributing improvements section to the bundled glab agent skill, inspired by the equivalent section in gitlab-org/ai/skills.

It also adds metadata.source-project and metadata.source-path to the skill's front matter. The section uses them to point at the upstream copy, and to tell glab-installed copies apart from third-party ones, which glab skills update would otherwise overwrite.

The section asks the agent to check the copy on gitlab-org/cli's main before proposing a fix. It distinguishes content read from the running binary, installs made by glab skills install, and third-party copies. It warns that glab skills update glab replaces any glab/ in both standard locations, and gives a per-location reinstall command when another copy must be left alone.

How to verify

• go test ./internal/commands/skills/... passes.
• lefthook run pre-push passes.