fix: add validation for config and reply subjects

Address medium severity security issues:

- Validate repo names in config only allow alphanumeric, dash, underscore
  (prevents NATS subject injection via dots or wildcards)
- Validate repo URLs must start with git+https://, git+ssh://, or git+file://
- Validate ReplyTo field must start with "build.responses." to prevent
  publishing responses to arbitrary NATS subjects

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/builder/config.go

@@ -3,10 +3,23 @@ package builder
 import (
 	"fmt"
 	"os"
+	"regexp"
+	"strings"
 
 	"gopkg.in/yaml.v3"
 )
 
+// repoNameRegex validates repository names for safe use in NATS subjects.
+// Only allows alphanumeric, dashes, and underscores (no dots or wildcards).
+var repoNameRegex = regexp.MustCompile(`^[a-zA-Z0-9_-]+$`)
+
+// validURLPrefixes are the allowed prefixes for repository URLs.
+var validURLPrefixes = []string{
+	"git+https://",
+	"git+ssh://",
+	"git+file://",
+}
+
 // RepoConfig holds configuration for a single repository.
 type RepoConfig struct {
 	URL           string `yaml:"url"`
@@ -44,9 +57,27 @@ func (c *Config) Validate() error {
 	}
 
 	for name, repo := range c.Repos {
+		// Validate repo name for safe use in NATS subjects
+		if !repoNameRegex.MatchString(name) {
+			return fmt.Errorf("repo name %q contains invalid characters (only alphanumeric, dash, underscore allowed)", name)
+		}
+
 		if repo.URL == "" {
 			return fmt.Errorf("repo %q: url is required", name)
 		}
+
+		// Validate URL format
+		validURL := false
+		for _, prefix := range validURLPrefixes {
+			if strings.HasPrefix(repo.URL, prefix) {
+				validURL = true
+				break
+			}
+		}
+		if !validURL {
+			return fmt.Errorf("repo %q: url must start with git+https://, git+ssh://, or git+file://", name)
+		}
+
 		if repo.DefaultBranch == "" {
 			return fmt.Errorf("repo %q: default_branch is required", name)
 		}