chore(skills): point remaining skill docs at org/public/core

[viktormarinho]

What is this contribution about?

Core skills are now synced into the org/public/core mount (see public-sets.ts), which replaces the image-baked /mnt/skills/public for skill discovery. main already migrated the file-reading and slides docs; this finishes the job for the remaining agent-facing references in xlsx, pdf, docx, pptx and the slides-create comment. The Dockerfile bake and the _bin bare commands (slides-create, pptx-*) still run from /mnt/skills and are intentionally left as-is — retiring those is a separate executable-distribution change.

How to Test

1. grep -rn "/mnt/skills" packages/sandbox/image/skills/*/SKILL.md → no matches.
2. The only remaining /mnt/skills refs are the Dockerfile, _bin/* wrappers, and the skills-features.md tracking doc (all load-bearing / accurate).

Migration Notes

Docs-only change; takes effect for agents on the next sandbox image rebuild / skill-set sync. No DB migration, no behavior change.

Review Checklist

• PR title is clear and descriptive
• Changes are tested and working
• Documentation is updated (if needed)
• No breaking changes

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Summary by cubic

Update skill docs and inline comments to use the org/public/core mount instead of /mnt/skills/public for docx, pdf, pptx, xlsx, and slides-create. Docs-only; no behavior changes—remaining /mnt/skills refs in the Dockerfile and _bin wrappers are intentional.

^{Written for commit 9ada835. Summary will update on new commits.}