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/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