Add deterministic deduplication, rclone backend, and database purge command

- Implement deterministic blob hashing using double SHA256 of uncompressed
  plaintext data, enabling deduplication even after local DB is cleared
- Add Stat() check before blob upload to skip existing blobs in storage
- Add rclone storage backend for additional remote storage options
- Add 'vaultik database purge' command to erase local state DB
- Add 'vaultik remote check' command to verify remote connectivity
- Show configured snapshots in 'vaultik snapshot list' output
- Skip macOS resource fork files (._*) when listing remote snapshots
- Use multi-threaded zstd compression (CPUs - 2 threads)
- Add writer tests for double hashing behavior

internal/vaultik/vaultik.go

@@ -91,6 +91,11 @@ func (v *Vaultik) Context() context.Context {
 	return v.ctx
 }
 
+// SetContext sets the Vaultik's context (primarily for testing)
+func (v *Vaultik) SetContext(ctx context.Context) {
+	v.ctx = ctx
+}
+
 // Cancel cancels the Vaultik's context
 func (v *Vaultik) Cancel() {
 	v.cancel()
@@ -124,6 +129,12 @@ func (v *Vaultik) GetFilesystem() afero.Fs {
 	return v.Fs
 }
 
+// Outputf writes formatted output to stdout for user-facing messages.
+// This should be used for all non-log user output.
+func (v *Vaultik) Outputf(format string, args ...any) {
+	_, _ = fmt.Fprintf(v.Stdout, format, args...)
+}
+
 // TestVaultik wraps a Vaultik with captured stdout/stderr for testing
 type TestVaultik struct {
 	*Vaultik