Fix bd-179: Derive prefix from database filename when config missing

- Add dbPath field to SQLiteStorage to track database file path
- Create derivePrefixFromPath() helper to extract prefix from filename
- Update ID generation in CreateIssue() and generateBatchIDs() to use filename fallback
- Fix tests to explicitly set issue_prefix config for bd- prefixed tests

When config doesn't have issue_prefix set, bd now correctly derives it from
the database filename (e.g., wy-.db -> wy) instead of always defaulting to 'bd'.

Fixes: bd-179

internal/storage/sqlite/lazy_init_test.go

@@ -30,6 +30,11 @@ func TestLazyCounterInitialization(t *testing.T) {
 
 	ctx := context.Background()
 
+	// Set the issue prefix to "bd" for this test
+	if err := store.SetConfig(ctx, "issue_prefix", "bd"); err != nil {
+		t.Fatalf("Failed to set issue_prefix: %v", err)
+	}
+
 	// Create some issues with explicit IDs (simulating import)
 	existingIssues := []string{"bd-5", "bd-10", "bd-15"}
 	for _, id := range existingIssues {
@@ -182,6 +187,11 @@ func TestCounterInitializationFromExisting(t *testing.T) {
 
 	ctx := context.Background()
 
+	// Set the issue prefix to "bd" for this test
+	if err := store.SetConfig(ctx, "issue_prefix", "bd"); err != nil {
+		t.Fatalf("Failed to set issue_prefix: %v", err)
+	}
+
 	// Create issues with explicit IDs, out of order
 	explicitIDs := []string{"bd-5", "bd-100", "bd-42", "bd-7"}
 	for _, id := range explicitIDs {