Add a configurable verb for the post-turn completion line

Summary

The end-of-turn completion line (e.g. "cogitated for 12s") uses a hard-coded rotating list of whimsical verbs, with no way to customize it via settings.json.

Note: spinnerVerbs is not the right place for this — the spinner verb is present tense ("Working…" while the turn is live) and the completion verb is past tense ("worked for 12s" once the turn ends). They're semantically distinct and should stay as separate settings.

Request

Add a new settings.json key (e.g. completionVerbs) with the same { mode: "replace" | "append", verbs: [...] } shape as spinnerVerbs, controlling only the post-turn completion line.

Motivation

Users who set spinnerVerbs to a plain verb already signal a preference for non-whimsical text. A matching key for the completion line would let them opt out of the rotating verbs end-to-end.

Environment

Claude Code CLI, macOS