feat: replace log.Printf with structured slog logging

- Add --log-format flag (text/json) and --verbosity flag (debug/info/warn/error)
- Replace all log.Printf with slog.Info/Debug/Warn with structured key-value attrs
- Replace all log.Fatalf with slog.Error + os.Exit(1)
- Convert gitea/client.go warnings to slog.Warn
- Add comprehensive tests for logger initialization and level filtering

Closes #23
Partially addresses #32

gitea/client.go

@@ -10,7 +10,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"log"
+	"log/slog"
 	"net/http"
 	"net/url"
 	"strings"
@@ -296,14 +296,14 @@ func (c *Client) GetAllFilesInPath(ctx context.Context, owner, repo, path string
 		case "file":
 			content, err := c.GetFileContent(ctx, owner, repo, entry.Path)
 			if err != nil {
-				log.Printf("Warning: could not fetch file %s: %v", entry.Path, err)
+				slog.Warn("could not fetch file from patterns repo", "file", entry.Path, "error", err)
 				continue
 			}
 			results[entry.Path] = content
 		case "dir":
 			subResults, err := c.GetAllFilesInPath(ctx, owner, repo, entry.Path)
 			if err != nil {
-				log.Printf("Warning: could not recurse into %s: %v", entry.Path, err)
+				slog.Warn("could not recurse into directory", "dir", entry.Path, "error", err)
 				continue
 			}
 			for k, v := range subResults {