dsh-win32 v0.5.4

Field report #3 (#6, same tireless tester). The Git Bash preset silently fails with PTY shell exited during startup when the session runs in the default workspace-write sandbox. That is the measured MSYS restricted-token death, but nothing told the user. Now the preset picker descriptions state the mode requirement on both variants, and doctor warns about it on Windows with the sandboxed alternative spelled out. No behavior change, pure guidance.