refactor: major codebase improvements and test framework overhaul

This commit represents a comprehensive refactoring of the codebase focused on
improving code quality, testability, and maintainability.

Key improvements:
- Implement dependency injection and interface-based architecture
- Add comprehensive test framework with fixtures and test suites
- Fix all linting issues (errcheck, gosec, staticcheck, goconst, etc.)
- Achieve full EditorConfig compliance across all files
- Replace hardcoded test data with proper fixture files
- Add configuration loader with hierarchical config support
- Improve error handling with contextual information
- Add progress indicators for better user feedback
- Enhance Makefile with help system and improved editorconfig commands
- Consolidate constants and remove deprecated code
- Strengthen validation logic for GitHub Actions
- Add focused consumer interfaces for better separation of concerns

Testing improvements:
- Add comprehensive integration tests
- Implement test executor pattern for better test organization
- Create extensive YAML fixture library for testing
- Fix all failing tests and improve test coverage
- Add validation test fixtures to avoid embedded YAML in Go files

Build and tooling:
- Update Makefile to show help by default
- Fix editorconfig commands to use eclint properly
- Add comprehensive help documentation to all make targets
- Improve file selection patterns to avoid glob errors

This refactoring maintains backward compatibility while significantly
improving the internal architecture and developer experience.

internal/interfaces.go

@@ -0,0 +1,80 @@
+// Package internal defines focused interfaces following Interface Segregation Principle.
+package internal
+
+import (
+	"os"
+
+	"github.com/schollz/progressbar/v3"
+
+	"github.com/ivuorinen/gh-action-readme/internal/errors"
+)
+
+// MessageLogger handles informational output messages.
+type MessageLogger interface {
+	Info(format string, args ...any)
+	Success(format string, args ...any)
+	Warning(format string, args ...any)
+	Bold(format string, args ...any)
+	Printf(format string, args ...any)
+	Fprintf(w *os.File, format string, args ...any)
+}
+
+// ErrorReporter handles error output and reporting.
+type ErrorReporter interface {
+	Error(format string, args ...any)
+	ErrorWithSuggestions(err *errors.ContextualError)
+	ErrorWithContext(code errors.ErrorCode, message string, context map[string]string)
+	ErrorWithSimpleFix(message, suggestion string)
+}
+
+// ErrorFormatter handles formatting of contextual errors.
+type ErrorFormatter interface {
+	FormatContextualError(err *errors.ContextualError) string
+}
+
+// ProgressReporter handles progress indication and status updates.
+type ProgressReporter interface {
+	Progress(format string, args ...any)
+}
+
+// OutputConfig provides configuration queries for output behavior.
+type OutputConfig interface {
+	IsQuiet() bool
+}
+
+// ProgressManager handles progress bar creation and management.
+type ProgressManager interface {
+	CreateProgressBar(description string, total int) *progressbar.ProgressBar
+	CreateProgressBarForFiles(description string, files []string) *progressbar.ProgressBar
+	FinishProgressBar(bar *progressbar.ProgressBar)
+	FinishProgressBarWithNewline(bar *progressbar.ProgressBar)
+	UpdateProgressBar(bar *progressbar.ProgressBar)
+	ProcessWithProgressBar(
+		description string,
+		items []string,
+		processFunc func(item string, bar *progressbar.ProgressBar),
+	)
+}
+
+// OutputWriter combines message logging and progress reporting for general output needs.
+type OutputWriter interface {
+	MessageLogger
+	ProgressReporter
+	OutputConfig
+}
+
+// ErrorManager combines error reporting and formatting for comprehensive error handling.
+type ErrorManager interface {
+	ErrorReporter
+	ErrorFormatter
+}
+
+// CompleteOutput combines all output interfaces for backward compatibility.
+// This should be used sparingly and only where all capabilities are truly needed.
+type CompleteOutput interface {
+	MessageLogger
+	ErrorReporter
+	ErrorFormatter
+	ProgressReporter
+	OutputConfig
+}