Adds a new `gitlab` output style that wraps each task's output in GitLab
CI collapsible section markers. Section IDs are generated automatically
so that start and end markers always match and stay unique per
invocation — even when the same task runs multiple times in one job.
Options: `collapsed` (maps to GitLab's native `[collapsed=true]`) and
`error_only` (Task-level behavior, identical to `group.error_only`).
Also introduces `output-ci-auto` (taskrc + TASK_OUTPUT_CI_AUTO env var)
that auto-selects a CI-aware output style when a supported CI runner is
detected (currently `GITLAB_CI=true` → gitlab) and no output style is
explicitly configured. Keeps the Taskfile neutral so local devs are not
forced into CI-shaped output.
Refs #2806.
