Step 4: Garden core with Init, Open, Add and CLI commands.

Garden package ties manifest and store together. Supports adding
files (hashed and stored as blobs), directories (manifest-only),
and symlinks (target recorded). Paths under $HOME are stored as
~/... in the manifest for portability. CLI init and add commands
wired up via cobra.

8 tests covering init, open, add for all three entry types,
duplicate rejection, HashFile, and tilde path expansion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

garden/hasher.go

@@ -0,0 +1,19 @@
+package garden
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"os"
+)
+
+// HashFile computes the SHA-256 hash of the file at path and returns
+// the hex-encoded hash string.
+func HashFile(path string) (string, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return "", fmt.Errorf("hashing file: %w", err)
+	}
+	sum := sha256.Sum256(data)
+	return hex.EncodeToString(sum[:]), nil
+}