multi: Add session-scoped attachment upload flow

Add an attachment upload pipeline that streams photo/file data from iOS
to the paired terminal session over RPC, avoiding server-side message
persistence for binary payloads.

Implement begin/chunk/commit/cancel handlers in the CLI session manager,
with size limits and temp-file cleanup, and wire iOS composer UI controls
for photo/files pickers, attachment chips, and send gating for ready
attachments.

Fix attachment RPC decoding to support wrapped result payloads so upload
begin/chunk/commit responses are interpreted correctly.

Author: bhandras

cli/internal/session/manager.go

@@ -80,6 +80,12 @@ type Manager struct {
 	sessionActorClosed     chan struct{}
 
 	lastTerminalKeepAliveSkipAt time.Time
+
+	// attachmentUploads tracks in-flight and recently committed attachment
+	// uploads for session-scoped upload RPC methods.
+	attachmentMu          sync.Mutex
+	attachmentUploads     map[string]*attachmentUploadState
+	lastAttachmentPruneAt time.Time
 }
 
 // NewManager creates a new session manager.
@@ -106,6 +112,7 @@ func NewManager(cfg *config.Config, token string, debug bool) (*Manager, error)
 		stopCh:       make(chan struct{}),
 		// Permission requests are owned by the SessionActor.
 		sessionActorClosed: make(chan struct{}),
+		attachmentUploads:  make(map[string]*attachmentUploadState),
 	}, nil
 }