Implement database handshake protocol in RPC layer

- Add ExpectedDB field to RPC Request
- Server validates client's expected DB matches daemon's DB
- Return clear error on mismatch with both paths
- Old clients (no ExpectedDB) still work with warning
- Add Path() method to storage.Storage interface
- Tests verify cross-database connections rejected

Prevents database pollution when client connects to wrong daemon.

Amp-Thread-ID: https://ampcode.com/threads/T-c4454192-39c6-4c67-96a9-675cbfc4db92
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/storage.go

@@ -69,6 +69,9 @@ type Storage interface {
 
 	// Lifecycle
 	Close() error
+
+	// Database path (for daemon validation)
+	Path() string
 }
 
 // Config holds database configuration