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/rpc/protocol.go

@@ -42,6 +42,7 @@ type Request struct {
 	RequestID     string          `json:"request_id,omitempty"`
 	Cwd           string          `json:"cwd,omitempty"`            // Working directory for database discovery
 	ClientVersion string          `json:"client_version,omitempty"` // Client version for compatibility checks
+	ExpectedDB    string          `json:"expected_db,omitempty"`    // Expected database path for validation (absolute)
 }
 
 // Response represents an RPC response from daemon to client