fix: correct duplicate message reporting in Go implementation

- Add DocumentSkippedError custom error type to distinguish between skipped and stored documents
- Fix counter bug where skipped messages were incorrectly reported as "stored"
- Enhance status reporting to show "X skipped as duplicates" for better visibility
- Fix Rust implementation binary attachment handling to support all file types (images, PDFs, etc.)
- Update test scripts to use correct binary names (mail2couch-go, mail2couch-rs)
- Add comprehensive test configurations for implementation comparison

Before: "Summary: Processed 30 messages, stored 30 new messages" (misleading when all were duplicates)
After: "Summary: Processed 30 messages, stored 0 new messages" with detailed "Stored 0/30 messages from INBOX (30 skipped as duplicates)"

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

test-both-implementations.sh

@@ -36,7 +36,7 @@ build_implementations() {
     # Build Go implementation
     echo -e "${BLUE}  Building Go implementation...${NC}"
     cd go
-    go build -o mail2couch .
+    go build -o mail2couch-go .
     cd ..
     echo -e "${GREEN}  ✅ Go implementation built${NC}"
     
@@ -53,7 +53,7 @@ run_go() {
     echo -e "${BLUE}🦬 Running Go implementation...${NC}"
     cd go
     echo -e "${BLUE}  Using config: config-test-go.json${NC}"
-    ./mail2couch -c config-test-go.json
+    ./mail2couch-go -c config-test-go.json
     cd ..
     echo -e "${GREEN}✅ Go implementation completed${NC}"
 }
@@ -63,7 +63,7 @@ run_rust() {
     echo -e "${BLUE}🦀 Running Rust implementation...${NC}"
     cd rust
     echo -e "${BLUE}  Using config: config-test-rust.json${NC}"
-    ./target/release/mail2couch -c config-test-rust.json
+    ./target/release/mail2couch-rs -c config-test-rust.json
     cd ..
     echo -e "${GREEN}✅ Rust implementation completed${NC}"
 }