feat(r-lib): add r-cran-status skill

[gadenbuie]

Summary

• Adds r-lib/r-cran-status, a skill to look up an R package's CRAN submission/review status (queue, human review, waiting, archived) or its R CMD check results (OK/NOTE/WARN/ERROR per platform)
• Registers the skill in .claude-plugin/marketplace.json under the r-lib plugin
• Adds a short entry to r-lib/README.md

Test plan

• Verified against sample packages (submission status + check results lookups)

Token count

r-cran-status (166 lines, ~1,638 tokens)

Skill description: ~84 tokens

Look up an R package's live status on cran.r-project.org - submission/review state (queue, human review, waiting, archived, past version's fate) or R CMD check results (OK/NOTE/WARN/ERROR per platform). Use for "what's the CRAN status of X", "did X get accepted/rejected", "is X passing CRAN checks", "when was version Y archived".

File	Lines	~Tokens
SKILL.md	166	1,638