PwshLocator accepts any PATH hit, including the WindowsApps alias stub, and now gates every session launch #104

Found during the pre-release code review of v0.6.0..HEAD. Low severity, but the blast radius grew in #95.

The change in behaviour

Before #95, RunInstance.ResolvePwsh actually ran PowerShell to decide:

Process.Start("pwsh.exe", "-NoLogo -NoProfile -Command \"exit 0\"");
// cached "pwsh.exe" only if it exited cleanly, else fell back to powershell.exe

PwshLocator replaced that with a where.exe pwsh.exe PATH lookup, for a good reason: the probe paid a full PowerShell startup (hundreds of ms) on a path that runs during session restore, which is what #82 is about.

The trade-off was stated in the code as "a broken-but-on-PATH pwsh surfaces as a failed session rather than a slower launch for everyone."

Why that trade-off is worse than it reads

where.exe proves a name resolves, not that it runs. The common false positive on Windows is a Microsoft Store App Execution Alias stub in %LOCALAPPDATA%\Microsoft\WindowsApps — a zero-byte reparse point that resolves on PATH and fails on execution when the app isn't installed.

And #95 didn't just change the run-command path. PwshLocator now also backs PseudoTerminal.BuildCmdLine, which wraps every non-shell session command. On a machine with that stub, the old code degraded to powershell.exe and everything worked; the new code picks pwsh.exe and every claude session fails to launch.

That is a cliff, not a degradation — and it's silent until someone tries to start a session.

Suggested fix

Keep the fast path, add a cheap sanity check on the resolved path rather than restoring the probe:

• where.exe gives the resolved path — check it's a real file with non-zero length (the alias stubs are zero-byte), or check for the reparse-point attribute.
• Or: keep where.exe, but fall back to powershell.exe on the first launch failure and remember it, so the cliff self-heals after one bad session instead of persisting for the process lifetime.

Either keeps the startup win from #95 without the failure mode.