fix: surface xcrun stderr when simctl list fails

[chroxify]

Problem

listDevicesInSet reads stdout with .Output() so the JSON stays clean, but on failure it wraps only the exit code, discarding the stderr that exec.ExitError already captured.

Every other simctl call in simctl.go (boot, shutdown, rename, …) uses CombinedOutput() and appends the output to its error. list is the one exception, and it's the first command most users run.

Repro

On a Mac whose developer directory points at the Command Line Tools (which don't ship simctl):

sudo xcode-select -s /Library/Developer/CommandLineTools
simslim list

Before:

simslim: simctl list: exit status 72

After:

simslim: simctl list: exit status 72: xcrun: error: unable to find utility "simctl",
not a developer tool or in PATH (simctl ships with full Xcode, not the Command Line
Tools; select one with `sudo xcode-select -s /Applications/Xcode.app`)

Change

• listError pulls stderr out of exec.ExitError and appends it, matching the file's existing simctl <verb>: %w: <output> style.
• When stderr shows xcrun can't find simctl at all, the error adds a one-line hint to select a full Xcode via xcode-select.
• TestFormatListError: pure table-driven unit test (no simulators booted), covering empty stderr, ordinary stderr, the missing-simctl hint, and errors.Is wrapping.

[Interlap01]

Thank you!