feat: add context lines for preview mode with intelligent truncation

- Add --context-lines (-c) flag to control preview context display
- Add context_lines option to .shush.toml configuration
- Implement lazy-git style preview with context lines around changes
- Add intelligent truncation with "..." markers for distant changes
- Improve context line colors from dark gray to light for better visibility on dark terminals
- Support both regular and git-aware preview modes
- Default to 3 context lines, configurable via flag or config file
- Use -c 0 to show only changed lines (original behavior)

internal/types/types.go

@@ -11,6 +11,7 @@ type CLI struct {
 	Backup        bool             `help:"Create backup files before modification"`
 	Verbose       bool             `help:"Show detailed output"`
 	PreserveLines bool             `help:"Remove comments but preserve empty lines"`
+	ContextLines  int              `short:"c" help:"Number of context lines to show in preview mode (default: from config)" default:"-1"`
 	LLM           bool             `help:"Show LLM-friendly usage guide"`
 	ChangesOnly   bool             `help:"Remove comments only from git changes (staged + unstaged + untracked)"`
 	Staged        bool             `help:"Remove comments only from staged git changes"`