Add Claude related folders/files to .gitignore (#4252)

## Motivation

For those of us using Claude Code, we probably don't want these files to
be committed as they can be specific to each of our setups

## Proposal

Add Claude files to `.gitignore`

## Test Plan

Files don't show up anymore

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

Author: ndr-ds

.gitignore

@@ -31,3 +31,7 @@ node_modules
 # as a library, so we don't want to
 # include their lock files.
 linera-web/**/*-lock.json
+
+# Claude
+.claude
+CLAUDE.md