Fix context propagation lifecycle bugs

Critical fixes to context propagation implementation (bd-rtp, bd-yb8, bd-2o2):

1. Fix rootCtx lifecycle in main.go:
   - Removed premature defer rootCancel() from PersistentPreRun (line 132)
   - Added proper cleanup in PersistentPostRun (lines 544-547)
   - Context now properly spans from setup through command execution to cleanup

2. Fix test context contamination in cli_fast_test.go:
   - Reset rootCtx and rootCancel to nil in test cleanup (lines 139-140)
   - Prevents cancelled contexts from affecting subsequent tests

3. Fix export tests missing context in export_test.go:
   - Added rootCtx initialization in 5 export test subtests
   - Tests now properly set up context before calling exportCmd.Run()

These fixes ensure:
- Signal-aware contexts work correctly for graceful cancellation
- Ctrl+C properly cancels import/export operations
- Database integrity is maintained after cancellation
- All cancellation tests pass (TestImportCancellation, TestExportCommand)

Tested:
- go build ./cmd/bd ✓
- go test ./cmd/bd -run TestImportCancellation ✓
- go test ./cmd/bd -run TestExportCommand ✓

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/cli_fast_test.go

@@ -135,6 +135,9 @@ func runBDInProcess(t *testing.T, dir string, args ...string) string {
 		flushTimer.Stop()
 		flushTimer = nil
 	}
+	// Reset context state
+	rootCtx = nil
+	rootCancel = nil
 	
 	// Give SQLite time to release file locks before cleanup
 	time.Sleep(10 * time.Millisecond)