Harden wsl.exe/git argument construction: leading-dash distro names, worktree path quoting #123

Two small argument-construction gaps left open by #121. Neither is reachable through normal use; both were found during that PR's review and deliberately parked rather than fixed, to keep the change focused.

1. A distro name beginning with - is not defended against

Documented under WSL Sessions → Known gaps in CLAUDE.md.

ShellSession.QuoteForCmd only wraps a value in quotes when it contains a space, tab or quote, so a distro named --help (or anything starting with -) reaches wsl.exe as a bare token and is parsed as an option rather than as the argument to -d. Distro names come from wsl -l -v, so this needs a user who imported a distro with a hostile name — low likelihood, but the value flows from state.json, which CLAUDE.md treats as untrusted input.

Note that quoting alone does not fix it: wsl.exe -d "-x" still parses as an option. The real fix is either rejecting such names in WslDiscoveryService.Parse or validating in LaunchValidationError, which already guards a blank distro on the same path.

2. GitService.CreateWorktreeAsync still uses naive quoting

GitService.cs:150-152 builds its arguments with plain \"{value}\" interpolation while everything else on the WSL path goes through QuoteForCmd. Since #121 routes git for \wsl$ folders through wsl.exe, that string now reaches one more layer than it used to. Inputs are user-typed branch names and paths rather than persisted state, so this is robustness rather than a security hole — but it is the last caller not using the shared helper, and the asymmetry is the kind that gets copied.

Verify

Unit tests only; both are covered by the existing Win32CommandLineTests round-trip harness, which tokenizes through the real CommandLineToArgvW.