fix: add comment for signature-agent (#73)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Clarifies the `--signature-agent` flag help for
`extensionsBuildWebBotAuthCmd` to specify it is a base URL and that
verifiers resolve `/.well-known/http-message-signatures-directory` from
it.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
d725950. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: archandatta

cmd/extensions.go

@@ -479,5 +479,5 @@ func init() {
 	extensionsBuildWebBotAuthCmd.Flags().String("url", "http://127.0.0.1:10001", "Base URL for update.xml and policy templates")
 	extensionsBuildWebBotAuthCmd.Flags().String("key", "", "Path to Ed25519 private key file (JWK or PEM format)")
 	extensionsBuildWebBotAuthCmd.Flags().String("upload", "", "Upload extension to Kernel with specified name (e.g., --upload web-bot-auth)")
-	extensionsBuildWebBotAuthCmd.Flags().String("signature-agent", "", "URL of the signature agent")
+	extensionsBuildWebBotAuthCmd.Flags().String("signature-agent", "", "Base URL of the signature agent (e.g., https://agent.example.com). Verifiers will look up /.well-known/http-message-signatures-directory at this URL.")
 }