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.

cmd/bd/migrate.go

@@ -356,6 +356,22 @@ This command:
 						color.Green("✓ Migrated %d issues to hash-based IDs\n", len(mapping))
 					}
 				}
+				
+				// Set id_mode=hash after successful migration (not in dry-run)
+				if !dryRun {
+					store, err := sqlite.New(targetPath)
+					if err == nil {
+						ctx := context.Background()
+						if err := store.SetConfig(ctx, "id_mode", "hash"); err != nil {
+							if !jsonOutput {
+								fmt.Fprintf(os.Stderr, "Warning: failed to set id_mode=hash: %v\n", err)
+							}
+						} else if !jsonOutput {
+							color.Green("✓ Switched database to hash ID mode\n")
+						}
+						store.Close()
+					}
+				}
 			} else {
 				store.Close()
 				if !jsonOutput {

cmd/bd/renumber_test.go

@@ -44,6 +44,7 @@ func TestRenumberWithGaps(t *testing.T) {
 
 	for _, tc := range testIssues {
 		issue := &types.Issue{
+			ID:          tc.id, // Set explicit ID to simulate gaps
 			Title:       tc.title,
 			Description: "Test issue for renumbering",
 			Priority:    1,
@@ -53,10 +54,6 @@ func TestRenumberWithGaps(t *testing.T) {
 		if err := testStore.CreateIssue(ctx, issue, "test"); err != nil {
 			t.Fatalf("failed to create issue: %v", err)
 		}
-		// Manually update ID to simulate gaps
-		if err := testStore.UpdateIssueID(ctx, issue.ID, tc.id, issue, "test"); err != nil {
-			t.Fatalf("failed to set issue ID to %s: %v", tc.id, err)
-		}
 	}
 
 	// Add a dependency to test that it gets updated