docs: fix dead GLM-Image skill link in master skill catalog

[siye566]

What

One line in skills/glm-master-skill/SKILL.md (GLM-Image catalog table): the skill name glm-image-gen and its link https://github.com/zai-org/GLM-Image/tree/main/skills/glm-image-gen both point to a path that returns 404. Corrected to glm-image-generation / https://github.com/zai-org/GLM-Image/tree/main/skills/glm-image-generation.

Why

The master skill is a navigation catalog; users following this row clone a path that does not exist. The correct name is confirmed by three sources:

• zai-org/GLM-Image/skills/ contains only glm-image-generation (verified via GitHub contents API);
• that skill's own SKILL.md frontmatter declares name: glm-image-generation;
• the same file already uses the correct name in its install commands (npx clawhub@latest install glm-image-generation, lines 77 and 83), so the catalog table was the only stale spot.

How it was checked

• Old link: HTTP 404; new link: verified reachable (200).
• Applicable pre-commit hooks checked manually (trailing whitespace, end-of-file newline, line endings, merge-conflict markers) — all clean.
• Documentation-only change; per the PR template's contribution guide, the remaining checks are skipped for doc fixes.