fix: address PR comments on argument parsing and README examples

- Use flag.NArg() and flag.Arg() for cleaner argument validation
- Call flag.Usage() on error instead of duplicating the message
- Use full package path in go tool examples for consistency

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

Author: kalbasit

README.md

@@ -58,13 +58,13 @@ At least one `--engine` flag is required; the tool exits with an error if none a
 SQLite + PostgreSQL only:
 
 ```bash
-go tool sqlc-multi-db --engine sqlite:sqlitedb --engine postgres:postgresdb postgresdb/querier.go
+go tool github.com/kalbasit/sqlc-multi-db --engine sqlite:sqlitedb --engine postgres:postgresdb postgresdb/querier.go
 ```
 
 All three engines:
 
 ```bash
-go tool sqlc-multi-db --engine sqlite:sqlitedb --engine postgres:postgresdb --engine mysql:mysqldb postgresdb/querier.go
+go tool github.com/kalbasit/sqlc-multi-db --engine sqlite:sqlitedb --engine postgres:postgresdb --engine mysql:mysqldb postgresdb/querier.go
 ```
 
 ### go:generate
@@ -74,7 +74,7 @@ Add a `generate.go` file in your database package (e.g., `pkg/database/generate.
 ```go
 package database
 
-//go:generate go tool sqlc-multi-db --engine sqlite:sqlitedb --engine postgres:postgresdb postgresdb/querier.go
+//go:generate go tool github.com/kalbasit/sqlc-multi-db --engine sqlite:sqlitedb --engine postgres:postgresdb postgresdb/querier.go
 ```
 
 Then run:

main.go

@@ -46,16 +46,12 @@ func main() {
 
 	flag.Parse()
 
-	if len(engines) == 0 {
-		log.Fatalf("USAGE: sqlc-multi-db --engine name:package [--engine ...] /path/to/querier.go")
+	if len(engines) == 0 || flag.NArg() != 1 {
+		flag.Usage()
+		os.Exit(1)
 	}
 
-	args := flag.Args()
-	if len(args) == 0 {
-		log.Fatalf("USAGE: sqlc-multi-db --engine name:package [--engine ...] /path/to/querier.go")
-	}
-
-	querierPath := args[0]
+	querierPath := flag.Arg(0)
 
 	if _, err := os.Stat(querierPath); err != nil {
 		log.Fatalf("stat(%q): %s", querierPath, err)