feat(depot): backend interface and local backend

Implement Backend interface with Probe/Get/Put methods, ProbeState constants,
and LocalBackend using filesystem storage with atomic operations and hash
verification.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/depot/backend.go

@@ -0,0 +1,24 @@
+package depot
+
+import "context"
+
+type ProbeState int
+
+const (
+	Present ProbeState = iota
+	Absent
+	Unconfirmed
+)
+
+// Backend defines the interface for blob storage backends.
+type Backend interface {
+	// Name returns the backend name.
+	Name() string
+	// Probe returns the existence state of one sha. transient=true means a
+	// retry might change the answer (feeds the serial confirm loop).
+	Probe(ctx context.Context, sha string) (state ProbeState, transient bool, err error)
+	// Get fetches sha into dest (temp file + rename), re-hashes, deletes on mismatch.
+	Get(ctx context.Context, sha, dest string) error
+	// Put uploads bytes from src for sha. Read-only backends return an error.
+	Put(ctx context.Context, sha, src string) error
+}