feat: add zeroclaw config schema for JSON Schema export

Add a `config schema` subcommand that dumps the full configuration
schema as JSON Schema (draft 2020-12) to stdout. This enables
downstream consumers (like PankoAgent) to programmatically validate
configs, generate forms, and stay in sync with zeroclaw's evolving
config surface without hand-maintaining copies of the schema.

- Add schemars 1.2 dependency and derive JsonSchema on all config
  structs/enums (schema.rs, policy.rs, email_channel.rs)
- Add `Config` subcommand group with `Schema` sub-command
- Output is valid JSON Schema with $defs for all 56 config types

src/channels/email_channel.rs

@@ -20,6 +20,7 @@ use lettre::{Message, SmtpTransport, Transport};
 use mail_parser::{MessageParser, MimeHeaders};
 use rustls::{ClientConfig, RootCertStore};
 use rustls_pki_types::DnsName;
+use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use std::collections::HashSet;
 use std::sync::Arc;
@@ -35,7 +36,7 @@ use uuid::Uuid;
 use super::traits::{Channel, ChannelMessage, SendMessage};
 
 /// Email channel configuration
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
 pub struct EmailConfig {
     /// IMAP server hostname
     pub imap_host: String,