Fix bd-270: Detect and handle Git merge conflicts in JSONL auto-import

- Add pre-parse merge conflict marker detection before JSON parsing
- Show clear error message when conflicts are detected
- Provide resolution instructions (Git client or bd export)
- Add TestAutoImportMergeConflict test case
- Prevents silent auto-import failures from cryptic parse errors

Amp-Thread-ID: https://ampcode.com/threads/T-d83011c0-7dfc-49c9-96c1-05c94ec2a3d3
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bufio"
+	"bytes"
 	"context"
 	"crypto/sha256"
 	"encoding/hex"
@@ -208,6 +209,22 @@ func autoImportIfNewer() {
 		fmt.Fprintf(os.Stderr, "Debug: auto-import triggered (hash changed)\n")
 	}
 
+	// Check for Git merge conflict markers (bd-270)
+	conflictMarkers := []string{"<<<<<<< ", "=======", ">>>>>>> "}
+	for _, marker := range conflictMarkers {
+		if bytes.Contains(jsonlData, []byte(marker)) {
+			fmt.Fprintf(os.Stderr, "\n❌ Git merge conflict detected in %s\n\n", jsonlPath)
+			fmt.Fprintf(os.Stderr, "The JSONL file contains unresolved merge conflict markers.\n")
+			fmt.Fprintf(os.Stderr, "This prevents auto-import from loading your issues.\n\n")
+			fmt.Fprintf(os.Stderr, "To resolve:\n")
+			fmt.Fprintf(os.Stderr, "  1. Resolve the merge conflict in your Git client, OR\n")
+			fmt.Fprintf(os.Stderr, "  2. Export from database to regenerate clean JSONL:\n")
+			fmt.Fprintf(os.Stderr, "     bd export -o %s\n\n", jsonlPath)
+			fmt.Fprintf(os.Stderr, "After resolving, commit the fixed JSONL file.\n")
+			return
+		}
+	}
+
 	// Content changed - parse all issues
 	scanner := bufio.NewScanner(strings.NewReader(string(jsonlData)))
 	scanner.Buffer(make([]byte, 0, 1024), 2*1024*1024) // 2MB buffer for large JSON lines