fix: simplify RunRemote to not re-prefix source or dest

RunRemote was adding user@host: prefixes on top of the paths already
pre-prefixed by engine.go in the remote-to-remote branch, causing
"both remote" rsync errors. The source path was also incorrectly
prefixed with srcUserHost:, making rsync reject the command entirely.

The fix: buildArgs already orders args correctly for rsync (source
then dest), so just use args[-2] and args[-1] as-is without any
additional prefixing.

Makefile

@@ -1,5 +1,5 @@
 BINARY=syncserver
-VERSION?=1.0.38
+VERSION?=1.0.39
 GO?=go
 LDFLAGS=-s -w -X main.version=$(VERSION) -X main.commit=$(shell git rev-parse --short HEAD 2>/dev/null || echo unknown)
 BUILD_FLAGS=CGO_ENABLED=0