fix(cli): mark Identity.hasPrivateKey @JsonIgnore

Jackson was writing the computed \`hasPrivateKey\` into identity.json
and then refusing to read it back ("Unrecognized field hasPrivateKey")
on the next invocation. All amy commands that reload the data-dir
(\`marmot group create\`, \`marmot key-package publish\`, …) exited 1 at
the Context.open step. Pure boolean getter, no persistence needed.

https://claude.ai/code/session_01M6dCKAF5Y1VyHGZPjzwDXq

cli/src/main/kotlin/com/vitorpamplona/amethyst/cli/Config.kt

@@ -45,6 +45,7 @@ data class Identity(
     val nsec: String?,
     val npub: String,
 ) {
+    @get:com.fasterxml.jackson.annotation.JsonIgnore
     val hasPrivateKey: Boolean get() = privKeyHex != null
 
     fun keyPair(): KeyPair =