feat(params): add structured argument binding helper

Add params.Bind, which unmarshals a tool call's map[string]any args into a
typed struct via JSON round-trip so JSON numbers land in the correct Go
numeric field types, and enforces `required:"true"` struct tags with clear
errors. Migrate the branch, tree, and file repo handlers to use it instead
of repeated args["x"].(string)/!ok extraction, preserving existing
validation behavior for each field.

Co-Authored-By: Codet <codet@commitgo.dev> (GPT-5-Codex)

pkg/params/bind.go

@@ -0,0 +1,55 @@
+package params
+
+import (
+	"encoding/json"
+	"fmt"
+	"reflect"
+	"strings"
+)
+
+// Bind decodes args into out, a pointer to a struct, replacing the repeated
+// args["x"].(string)/!ok extraction pattern. It round-trips args through JSON
+// so JSON numbers land in the correct Go numeric field types.
+//
+// Struct fields tagged `required:"true"` must be present in args; a missing
+// key, or an empty string value for a string field, returns an error naming
+// the field's json tag.
+func Bind(args map[string]any, out any) error {
+	v := reflect.ValueOf(out)
+	if v.Kind() != reflect.Pointer || v.Elem().Kind() != reflect.Struct {
+		return fmt.Errorf("params.Bind: out must be a pointer to a struct, got %T", out)
+	}
+
+	if err := checkRequiredFields(args, v.Elem().Type()); err != nil {
+		return err
+	}
+
+	data, err := json.Marshal(args)
+	if err != nil {
+		return fmt.Errorf("params.Bind: marshal args: %w", err)
+	}
+	if err := json.Unmarshal(data, out); err != nil {
+		return fmt.Errorf("params.Bind: %w", err)
+	}
+	return nil
+}
+
+func checkRequiredFields(args map[string]any, t reflect.Type) error {
+	for field := range t.Fields() {
+		if field.Tag.Get("required") != "true" {
+			continue
+		}
+		name, _, _ := strings.Cut(field.Tag.Get("json"), ",")
+		if name == "" || name == "-" {
+			continue
+		}
+		val, ok := args[name]
+		if !ok {
+			return fmt.Errorf("%s is required", name)
+		}
+		if s, isString := val.(string); isString && s == "" {
+			return fmt.Errorf("%s is required", name)
+		}
+	}
+	return nil
+}