add generated jsonschema for config.toml (#8956)

### What
Add JSON Schema generation for `config.toml`, with checked‑in
`docs/config.schema.json`. We can move the schema elsewhere if preferred
(and host it if there's demand).

Add fixture test to prevent drift and `just write-config-schema` to
regenerate on schema changes.

Generate MCP config schema from `RawMcpServerConfig` instead of
`McpServerConfig` because that is the runtime type used for
deserialization.

Populate feature flag values into generated schema so they can be
autocompleted.

### Tests
Added tests + regenerate script to prevent drift. Tested autocompletions
using generated jsonschema locally with Even Better TOML.



https://github.com/user-attachments/assets/5aa7cd39-520c-4a63-96fb-63798183d0bc

codex-rs/core/src/features.rs

@@ -8,6 +8,7 @@
 use crate::config::ConfigToml;
 use crate::config::profile::ConfigProfile;
 use codex_otel::OtelManager;
+use schemars::JsonSchema;
 use serde::Deserialize;
 use serde::Serialize;
 use std::collections::BTreeMap;
@@ -15,6 +16,7 @@ use std::collections::BTreeSet;
 
 mod legacy;
 pub(crate) use legacy::LegacyFeatureToggles;
+pub(crate) use legacy::legacy_feature_keys;
 
 /// High-level lifecycle stage for a feature.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -294,7 +296,7 @@ pub fn is_known_feature_key(key: &str) -> bool {
 }
 
 /// Deserializable features table for TOML.
-#[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq)]
+#[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, JsonSchema)]
 pub struct FeaturesToml {
     #[serde(flatten)]
     pub entries: BTreeMap<String, bool>,