[New] Exclude tests and dev files from release tarballs

[New] .gitattributes export-ignore for tests/, docs/, pkg/, .github/, dev files
[Change] pkg/Makefile tar fallback: add --exclude for tests/ and .github/

Author: rfxn

.gitattributes

@@ -0,0 +1,11 @@
+# Exclude from release tarballs (git archive / GitHub downloads)
+tests/              export-ignore
+docs/               export-ignore
+pkg/                export-ignore
+audit-output/       export-ignore
+.github/            export-ignore
+CLAUDE.md           export-ignore
+PLAN*.md            export-ignore
+AUDIT.md            export-ignore
+HANDOFF.md          export-ignore
+GEMINI-review.md    export-ignore

pkg/Makefile

@@ -42,8 +42,8 @@ $(BUILDDIR)/$(TARBALL):
 	    cd $(PROJDIR) && \
 	    tar czf $(BUILDDIR)/$(TARBALL) \
 	        --transform 's,^\./,$(NAME)-$(VERSION)/,' \
-	        --exclude='.git' --exclude='audit-output' \
-	        --exclude='.claude' \
+	        --exclude='.git' --exclude='tests' --exclude='audit-output' \
+	        --exclude='.claude' --exclude='.github' \
 	        --exclude='CLAUDE.md' --exclude='PLAN*.md' \
 	        --exclude='AUDIT.md' --exclude='MEMORY.md' \
 	        --exclude='pkg/build' \