Guest session cwd and command preflight helpers were emitting literal
`\\n` separators, so the guest shell saw malformed one-line scripts and
could fail `preflight_cwd` even when `/root/repo` already existed.
Replace those builders with real newlines, and fix the nearby attach
helper commands that were making the same mistake.
Add a small daemon guest-SSH seam so workspace preparation and session
start can share a fake backend in tests, then cover the regression with
an end-to-end daemon test for `PrepareVMWorkspace` followed by
`StartGuestSession` on `/root/repo`.
Validation: `GOCACHE=/tmp/banger-gocache go test ./internal/daemon` and
`GOCACHE=/tmp/banger-gocache go test ./...`.
