exit 0 when the model id is unknown

[andredezzy]

Problem

run throws ExitCode(1) when --model names an id that is not in the
registry. The LaunchAgent written by parrot install --launch-at-login sets
KeepAlive{SuccessfulExit: false}, so launchd relaunches the daemon, which
prints the same error and exits again.

Observed on macOS 26.4.1 after pointing the agent at an id that did not exist:

unknown model: whisper-large-v3-turbo-compressed
run `parrot models list` to see options.
unknown model: whisper-large-v3-turbo-compressed
run `parrot models list` to see options.
unknown model: whisper-large-v3-turbo-compressed

A wrong id in a plist cannot fix itself by being retried, so every relaunch is
guaranteed to fail the same way. Dictation is silently dead while launchd spins.

Fix

Exit 0. The user still gets the message and the pointer to parrot models list,
once instead of on a loop.

Relationship to the accessibility path

This is the same defect in a second place — a non-transient failure exiting
nonzero under KeepAlive. The accessibility case is the one users are more
likely to hit (a new release invalidates the TCC grant, since the shipped
binaries are adhoc, linker-signed and TCC then anchors to the cdhash), and it
produced nine permission dialogs in a row on this machine. Both fixes are
one-liners and independent; either can land alone.

Built locally with swift build -c release (no PR CI in this repo); clean, no
warnings.