fix(claude): omit --model in completeOneShot when alias resolves empty

[Antisophy]

Problem

completeOneShot (title generation) passes --model resolveModelAlias(modelClass) unconditionally. An empty model_aliases entry makes resolveModelAlias return "", so it runs claude --model "" and fails with API Error 400 ("String should have at least 1 character"), surfacing as failed to generate title: claude exited with status 1. The session-spawn path already guards this (config.model.length > 0); completeOneShot is the one spot that doesn't.

Motivation

After Fable 5 was shut down I wanted cydo to stop pinning a model and instead defer to claude's own configured default, so it would fall back gracefully rather than stay pointed at an unavailable model. The way to express that is empty model_aliases entries, but that config wasn't actually runnable: completeOneShot passed --model "" and broke title generation on the next cydo task I started. This PR's change is what makes the empty-alias / defer-to-claude config work end to end.

Fix

Append --model only when the resolved model is non-empty (matching the session-spawn guard), and collapse the duplicated cmdPrefix / non-cmdPrefix arg lists into one.

[Antisophy]

Split into two commits so the behavior fix stands on its own: the first commit is just the --model omission, and the second extracts the argument construction into a pure static buildOneShotArgs and adds the unit test.

Reason for the extraction: completeOneShot spawns the claude process, so its argument list can't be observed from a unit test in place. Pulling the arg-building into a pure function lets the test assert that an empty alias omits --model (and a real alias still passes --model <x>) without launching anything. The extraction is behavior-preserving; the fix commit plus the refactor produce exactly the tree that was here before.