T1.3: Create src/runner; move runner logic

Move runner.go, runner_windows.go, runner_other.go, runner_test.go from
src/core to src/runner (package runner). Split helpers into focused files:
invocation*.go, exitcodes.go, logfile.go, cleanup.go. Export LogArguments
and SuccessExitCodesText for use by scheduler's runningOutput helper.
Update src/core/scheduler.go and src/gui/app.go to import src/runner.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/runner/invocation_windows.go

@@ -0,0 +1,58 @@
+package runner
+
+import (
+	"context"
+	"os/exec"
+	"strings"
+	"syscall"
+	"unicode"
+)
+
+func shellCommand(ctx context.Context, command string) *exec.Cmd {
+	// cmd.exe keeps Windows users' expectations for commands such as "dir",
+	// "copy", variable expansion, redirection, and .bat/.cmd wrappers.
+	//
+	// Go's normal Windows argument escaping turns embedded quotes into literal
+	// backslash-quote sequences for cmd.exe. Supplying the raw command line keeps
+	// commands like `"C:\Program Files\App\App.exe" "D:\file.txt"` executable.
+	result := exec.CommandContext(ctx, "cmd.exe")
+	result.SysProcAttr = &syscall.SysProcAttr{CmdLine: windowsShellCommandLine(command)}
+	return result
+}
+
+func windowsShellCommandLine(command string) string {
+	return `cmd.exe /S /C "` + quoteLeadingWindowsProgramPath(command) + `"`
+}
+
+func quoteLeadingWindowsProgramPath(command string) string {
+	trimmed := strings.TrimLeftFunc(command, unicode.IsSpace)
+	leadingWhitespace := command[:len(command)-len(trimmed)]
+	if trimmed == "" || strings.HasPrefix(trimmed, `"`) || !startsWithWindowsRootedPath(trimmed) {
+		return command
+	}
+
+	lower := strings.ToLower(trimmed)
+	for _, extension := range []string{".exe", ".cmd", ".bat", ".com"} {
+		index := strings.Index(lower, extension)
+		if index < 0 {
+			continue
+		}
+		pathEnd := index + len(extension)
+		programPath := trimmed[:pathEnd]
+		if !strings.ContainsFunc(programPath, unicode.IsSpace) {
+			return command
+		}
+		return leadingWhitespace + `"` + programPath + `"` + trimmed[pathEnd:]
+	}
+	return command
+}
+
+func startsWithWindowsRootedPath(command string) bool {
+	if strings.HasPrefix(command, `\\`) {
+		return true
+	}
+	return len(command) >= 3 &&
+		((command[0] >= 'A' && command[0] <= 'Z') || (command[0] >= 'a' && command[0] <= 'z')) &&
+		command[1] == ':' &&
+		(command[2] == '\\' || command[2] == '/')
+}