Refactoring complete: v0.4.0 architectural milestone (#1)

## Summary

Completed Phase 5 refactoring and reached the target architecture.

**Architectural milestone achieved:**
- Service layer owns all state and is the sole writer
- UI is a thin Fyne view, all widget updates marshaled via `fyne.Do`
- Core engines are stateless and injectable
- Domain types are pure (no `yaml:"-"` fields)
- Full module builds and `go vet ./...` clean

## Changes

- Bump version: 0.3.6 → 0.4.0
- Update CHANGELOG with Phase 5 summary
- Add ROADMAP "Refactoring Follow-Ups" section

## Known follow-up work

1. **Linux test build broken** — `runner_test.go` needs `//go:build windows` tag
2. **File-size limits exceeded** — `operations.go` (486 lines), `jobs_view.go` (415 lines)

See ROADMAP.md for details.

---------

Co-authored-by: mixeme <mix.public@ya.ru>
Reviewed-on: #1

src/runner/cleanup.go

@@ -0,0 +1,61 @@
+package runner
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"time"
+)
+
+func CleanupLogs(logsDir string, maxFiles int, maxAgeDays int) error {
+	entries, err := os.ReadDir(logsDir)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil
+		}
+		return err
+	}
+
+	type logFile struct {
+		path    string
+		modTime time.Time
+	}
+	var logs []logFile
+	cutoff := time.Now().AddDate(0, 0, -maxAgeDays)
+	for _, entry := range entries {
+		// Only GoSentry run logs are managed here. Directories and non-.log files
+		// are intentionally ignored so the user can keep notes or other artifacts
+		// in the same folder without the cleanup policy deleting them.
+		if entry.IsDir() || !strings.HasSuffix(strings.ToLower(entry.Name()), ".log") {
+			continue
+		}
+		path := filepath.Join(logsDir, entry.Name())
+		info, err := entry.Info()
+		if err != nil {
+			continue
+		}
+		if maxAgeDays > 0 && info.ModTime().Before(cutoff) {
+			// Cleanup is best-effort: failing to delete one file should not block
+			// the scheduler from running future jobs.
+			_ = os.Remove(path)
+			continue
+		}
+		logs = append(logs, logFile{path: path, modTime: info.ModTime()})
+	}
+
+	if maxFiles <= 0 || len(logs) <= maxFiles {
+		return nil
+	}
+	sort.Slice(logs, func(i int, j int) bool {
+		// Newest files are kept first, then everything after maxFiles is removed.
+		// This matches the user's expectation that the most recent failures and
+		// command output remain available for investigation.
+		return logs[i].modTime.After(logs[j].modTime)
+	})
+	for _, old := range logs[maxFiles:] {
+		_ = os.Remove(old.path)
+	}
+	return nil
+}