fix: rsync double SSH spec in RunRemote, restore dropped flags in Run

- Fix Bug A: RunRemote was re-prefixing destination with user@host: when
  engine.go:135 already pre-prefixed it for remote-to-remote, causing
  "admin@host:admin@host:/path" to be passed to rsync
- Fix Bug B: Run used args[1:] when inserting -e ssh, silently dropping
  the first rsync flag (e.g. -aP became flag-less)
- Fix Bug C: RunRemote extracted source/dest in wrong order for pull
  direction (buildArgs reverses them but RunRemote assumed push order)
- Add rsync_runner_test.go covering buildArgs, RunRemote prefixing,
  and flag preservation

Makefile

@@ -1,5 +1,5 @@
 BINARY=syncserver
-VERSION?=1.0.36
+VERSION?=1.0.37
 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