add CLAUDE.md with project context for Claude Code (#261)

* add CLAUDE.md with project context for Claude Code

Provides development commands, architecture overview, file conventions,
custom S3 classes, data files, testing setup, and versioning guidelines
to help Claude Code assist effectively in this repository.

https://claude.ai/code/session_01L3fTxqsg3Gc6J1znpWN1Mx

* add CLAUDE.md to .Rbuildignore

Excludes the Claude Code context file from the R package build tarball.

https://claude.ai/code/session_01L3fTxqsg3Gc6J1znpWN1Mx

* document version-bump requirement for every PR in CLAUDE.md

Each PR must increment the .9zzz dev counter by 1 in both
DESCRIPTION (Version: field) and NEWS.md (top-level heading).

https://claude.ai/code/session_01L3fTxqsg3Gc6J1znpWN1Mx

---------

Co-authored-by: Claude <noreply@anthropic.com>

.Rbuildignore

@@ -9,6 +9,7 @@
 ^_pkgdown\.yml$
 ^appveyor\.yml$
 ^codecov\.yml$
+^CLAUDE\.md$
 ^cran-comments\.md$
 ^CRAN-RELEASE$
 ^\.github$