Add agent skill for dependency version updates

[bdice]

Description

Add an agent skill (.agents/skills/update-dependency-version/SKILL.md) that codifies the procedure for updating dependency versions in rapids-cmake/cpm/versions.json.

I used this skill to create #990 and other recent CCCL update PRs.

The skill documents:

• The two JSON schemas used in versions.json (git mode and URL/tarball mode)
• How to resolve commit SHAs from tags (including nested tag dereferencing) and branches
• How to compute SHA256 hashes for tarball-mode packages
• How to determine the upstream version string
• Branch, commit, PR, and label conventions

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[bdice]

@robertmaynard Curious for your thoughts on this when you have time.

[robertmaynard]

@bdice Fine with this. Feel free to merge whenever you want

[bdice]

/merge

[bdice]

@robertmaynard Can you approve so it's mergeable?