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/container/host_environment_test.go

@@ -6,12 +6,14 @@ package container
 
 import (
 	"archive/tar"
+	"bytes"
 	"context"
 	"io"
 	"os"
 	"path"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"testing"
 
 	"gitea.com/gitea/runner/act/common"
@@ -100,6 +102,45 @@ func TestHostEnvironmentExecExitCode(t *testing.T) {
 	assert.Equal(t, "Process completed with exit code 3.", err.Error())
 }
 
+func TestHostEnvironmentAllocatePTY(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("uses POSIX shell")
+	}
+	for _, tc := range []struct {
+		name     string
+		allocPTY bool
+		expect   string
+	}{
+		{name: "off", allocPTY: false, expect: "NOTTY"},
+		{name: "on", allocPTY: true, expect: "TTY"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			dir := t.TempDir()
+			buf := &bytes.Buffer{}
+			e := &HostEnvironment{
+				Path:        filepath.Join(dir, "path"),
+				TmpDir:      filepath.Join(dir, "tmp"),
+				ToolCache:   filepath.Join(dir, "tool_cache"),
+				ActPath:     filepath.Join(dir, "act_path"),
+				StdOut:      buf,
+				Workdir:     filepath.Join(dir, "path"),
+				AllocatePTY: tc.allocPTY,
+			}
+			for _, p := range []string{e.Path, e.TmpDir, e.ToolCache, e.ActPath} {
+				require.NoError(t, os.MkdirAll(p, 0o700))
+			}
+
+			err := e.Exec(
+				[]string{"sh", "-c", "[ -t 1 ] && printf TTY || printf NOTTY"},
+				map[string]string{"PATH": os.Getenv("PATH")}, "", "",
+			)(context.Background())
+			require.NoError(t, err)
+			got := strings.TrimSpace(strings.ReplaceAll(buf.String(), "\r", ""))
+			assert.Equal(t, tc.expect, got)
+		})
+	}
+}
+
 func TestHostEnvironmentRemoveCleansWorkdir(t *testing.T) {
 	logger := logrus.New()
 	ctx := common.WithLogger(context.Background(), logrus.NewEntry(logger))