feat(dealer): add keepElapsed option to persist elapsed time

[YusukeHirao]

Summary

• Add TaskListRunOptions.keepElapsed (default false) so done/error lines can retain the step's final elapsed time instead of losing it once the step settles
• keepElapsed is subordinate to showElapsed: when showElapsed: false, keepElapsed has no effect
• verbose mode still disables the running-step timer as before, but the settled-line elapsed time follows keepElapsed independently (single-shot output, no log flooding)

Test plan

• yarn lint
• yarn build (with NX_WORKSPACE_ROOT_PATH for the worktree)
• yarn test
• Added unit tests covering: default (no elapsed on settle), keepElapsed: true on done/error lines, subordination to showElapsed: false, persistence in verbose mode, and independence across multiple steps

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