Stop shipping broken agent-skill symlinks in the source release (#73107)

Excluding .agents from the source tarball (#68851) left .claude behind.
Everything under .claude/skills is a relay symlink into .agents/skills, so
export-ignore strips the targets while the links themselves still ship:
unpacking the source release yields broken symlinks, and .claude/ arrives
holding nothing but those dead links.

Found while verifying 3.3.2rc1, whose tarball carries five of them. The
released 3.3.1 carries the same ones, so this is long-standing rather than
something a recent change introduced.

.github needs no equivalent entry: its own skills relays are already
covered by the existing .github export-ignore.

.gitattributes

@@ -22,6 +22,7 @@ agents.md export-ignore
 .agents export-ignore
 SKILLS.md export-ignore
 CLAUDE.md export-ignore
+.claude export-ignore
 
 .asf.yaml export-ignore
 .bash_completion export-ignore