feat: make pseudo-TTY allocation opt-in (#961)

Fixes #956.

Pseudo-TTY allocation is now an explicit, runner-wide opt-in via `runner.allocate_pty`, applied to both host and docker backends. Default is off, matching GitHub `actions/runner`.

```yaml
runner:
  allocate_pty: false  # default
```

**Before:** the host backend hardcoded `if true /* allocate Terminal */` and the docker backend used `term.IsTerminal(os.Stdout.Fd())`. As a result, `docker build` (and other TTY-aware tools) saw a TTY and emitted cursor-control redraw frames that flooded captured logs with thousands of duplicate-looking progress lines — only on host-mode runners in production, and on docker-mode runners when the daemon happened to be launched from a shell rather than a service.

**After:** both backends consult `Config.AllocatePTY`. The `term.IsTerminal` heuristic is gone, so behavior no longer depends on whether the daemon has a controlling terminal.

**Reproduction:** running `docker build` through `HostEnvironment.Exec` with output captured to a buffer:

| | Before (`if true`) | After (`AllocatePTY=false`) |
|---|---:|---:|
| bytes captured | 18,167 | 1,048 |
| ANSI CSI sequences | 556 | 0 |
| cursor-up `\e[1A` | 181 | 0 |

**Side fix:** `ptyWriter.AutoStop` is now `atomic.Bool`. The field is written from the exec goroutine after `cmd.Wait()` and read from the `copyPtyOutput` goroutine via `ptyWriter.Write`; existing tests never tripped the race detector because their commands produced no output before exit. The new host-mode test does.

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/961
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-committed-by: silverwind <2021+silverwind@noreply.gitea.com>

act/runner/step_docker_test.go

@@ -109,6 +109,55 @@ func TestStepDockerMain(t *testing.T) {
 	cm.AssertExpectations(t)
 }
 
+func TestStepDockerNewStepContainerAllocatePTY(t *testing.T) {
+	for _, tc := range []struct {
+		name     string
+		allocPTY bool
+	}{
+		{name: "off", allocPTY: false},
+		{name: "on", allocPTY: true},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			cm := &containerMock{}
+
+			var captured *container.NewContainerInput
+			origContainerNewContainer := ContainerNewContainer
+			ContainerNewContainer = func(input *container.NewContainerInput) container.ExecutionsEnvironment {
+				captured = input
+				return cm
+			}
+			defer func() {
+				ContainerNewContainer = origContainerNewContainer
+			}()
+
+			ctx := context.Background()
+			sd := &stepDocker{
+				RunContext: &RunContext{
+					StepResults: map[string]*model.StepResult{},
+					Config: &Config{
+						AllocatePTY: tc.allocPTY,
+						PlatformPicker: func(_ []string) string {
+							return "node:14"
+						},
+					},
+					Run: &model.Run{
+						JobID: "1",
+						Workflow: &model.Workflow{
+							Jobs: map[string]*model.Job{"1": {}},
+						},
+					},
+					JobContainer: cm,
+				},
+				Step: &model.Step{ID: "1", Uses: "docker://node:14"},
+			}
+			sd.RunContext.ExprEval = sd.RunContext.NewExpressionEvaluator(ctx)
+
+			_ = sd.newStepContainer(ctx, "node:14", []string{"echo", "hi"}, nil)
+			assert.Equal(t, tc.allocPTY, captured.AllocatePTY)
+		})
+	}
+}
+
 func TestStepDockerPrePost(t *testing.T) {
 	ctx := context.Background()
 	sd := &stepDocker{}