Make hash IDs opt-in via id_mode config (bd-5404)

- Add id_mode config (sequential|hash), defaults to sequential
- Update CreateIssue/CreateIssues to check id_mode and generate appropriate IDs
- Implement lazy counter initialization from existing issues
- Update migrate --to-hash-ids to set id_mode=hash after migration
- Fix hash ID tests to set id_mode=hash
- Fix renumber test to use explicit IDs
- All 183 test packages pass

This makes hash IDs backward-compatible opt-in rather than forced default.

internal/storage/sqlite/hash_id_test.go

@@ -17,10 +17,13 @@ func TestHashIDGeneration(t *testing.T) {
 
 	ctx := context.Background()
 
-	// Set up database with prefix
+	// Set up database with prefix and hash mode
 	if err := store.SetConfig(ctx, "issue_prefix", "bd"); err != nil {
 		t.Fatalf("Failed to set prefix: %v", err)
 	}
+	if err := store.SetConfig(ctx, "id_mode", "hash"); err != nil {
+		t.Fatalf("Failed to set id_mode: %v", err)
+	}
 
 	// Create an issue - should get a hash ID
 	issue := &types.Issue{
@@ -139,10 +142,13 @@ func TestHashIDBatchCreation(t *testing.T) {
 
 	ctx := context.Background()
 
-	// Set up database with prefix
+	// Set up database with prefix and hash mode
 	if err := store.SetConfig(ctx, "issue_prefix", "bd"); err != nil {
 		t.Fatalf("Failed to set prefix: %v", err)
 	}
+	if err := store.SetConfig(ctx, "id_mode", "hash"); err != nil {
+		t.Fatalf("Failed to set id_mode: %v", err)
+	}
 
 	// Create multiple issues with similar content
 	issues := []*types.Issue{