✨ feat(output): add gitlab output mode (#2806)

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.

setup.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"path/filepath"
 	"slices"
+	"strconv"
 	"strings"
 	"sync"
 
@@ -202,12 +203,27 @@ func (e *Executor) setupOutput() error {
 	if !e.OutputStyle.IsSet() {
 		e.OutputStyle = e.Taskfile.Output
 	}
+	if !e.OutputStyle.IsSet() && e.OutputCIAuto {
+		if name := detectCIOutput(); name != "" {
+			e.OutputStyle.Name = name
+		}
+	}
 
 	var err error
 	e.Output, err = output.BuildFor(&e.OutputStyle, e.Logger)
 	return err
 }
 
+// detectCIOutput returns the name of a CI-aware output style to use based
+// on environment variables set by common CI runners. Returns an empty string
+// when no supported CI environment is detected.
+func detectCIOutput() string {
+	if isGitLab, _ := strconv.ParseBool(os.Getenv("GITLAB_CI")); isGitLab {
+		return "gitlab"
+	}
+	return ""
+}
+
 func (e *Executor) setupCompiler() error {
 	if e.UserWorkingDir == "" {
 		var err error