Add GitLab wiki URL resolver script

[benhunter]

Motivation

• Provide a small CLI helper to resolve the clone URL for a GitLab project or group wiki from a single GitLab URL.
• Support environments with or without glab by falling back to curl with GITLAB_TOKEN for API access.
• Give clear usage, error messages, and exit codes so callers can detect missing/disabled wikis programmatically.

Description

• Add gitlab-wiki-url.sh which validates and parses a single GitLab URL and normalizes the host and path.
• Query the GitLab API (base https://<host>/api/v4) using glab api when available or curl with PRIVATE-TOKEN: $GITLAB_TOKEN as a fallback via the api_get helper.
• Attempt GET /projects/:urlencoded_path to detect a project and check wiki_enabled, returning the repo URL with .wiki.git or exiting 2 if the wiki is disabled.
• If the project lookup 404s, attempt GET /groups/:urlencoded_path and then GET /groups/:id/wikis to determine a group wiki and return <group_web_url>.wiki.git, with distinct exit codes for not found, validation/API errors, and success.
• Update README.md with the new script entry, required environment variable note for GITLAB_TOKEN, and example invocations.

Testing

• No automated tests were run for this change.

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Codex Task