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
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