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

Cargo.toml

@@ -37,6 +37,9 @@ directories = "6.0"
 toml = "1.0"
 shellexpand = "3.1"
 
+# JSON Schema generation for config export
+schemars = "1.2"
+
 # Logging - minimal
 tracing = { version = "0.1", default-features = false }
 tracing-subscriber = { version = "0.3", default-features = false, features = ["fmt", "ansi", "env-filter"] }