Fix multiple issues and renumber database

- bd-170: Implement hybrid sorting for ready work (recent 48h first, then oldest)
- bd-87: Use safer null-byte placeholders in ID remapping
- bd-92: Make auto-flush debounce configurable via BEADS_FLUSH_DEBOUNCE
- bd-171: Fix nil pointer dereference in renumber command
- Delete spurious test issues (bd-7, bd-130-134)
- Renumber database from 171 down to 144 issues

cmd/bd/main_test.go

@@ -114,9 +114,8 @@ func TestAutoFlushDebounce(t *testing.T) {
 	storeMutex.Unlock()
 
 	// Set short debounce for testing (100ms)
-	originalDebounce := flushDebounce
-	flushDebounce = 100 * time.Millisecond
-	defer func() { flushDebounce = originalDebounce }()
+	os.Setenv("BEADS_FLUSH_DEBOUNCE", "100ms")
+	defer os.Unsetenv("BEADS_FLUSH_DEBOUNCE")
 
 	// Reset auto-flush state
 	autoFlushEnabled = true