[codex] Support local marketplace sources (#17756)

## Summary

- Port marketplace source support into the shared core marketplace-add
flow
- Support local marketplace directory sources
- Support direct `marketplace.json` URL sources
- Persist the new source types in config/schema and cover them in CLI
and app-server tests

## Validation

- `cargo test -p codex-core marketplace_add`
- `cargo test -p codex-cli marketplace_add`
- `cargo test -p codex-app-server marketplace_add`
- `just write-config-schema`
- `just fmt`
- `just fix -p codex-core`
- `just fix -p codex-cli`

## Context

Current `main` moved marketplace-add behavior into shared core code and
still assumed only git-backed sources. This change keeps that structure
but restores support for local directories and direct manifest URLs in
the shared path.

codex-rs/config/src/types.rs

@@ -632,6 +632,7 @@ pub struct MarketplaceConfig {
 #[serde(rename_all = "snake_case")]
 pub enum MarketplaceSourceType {
     Git,
+    Local,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Default, JsonSchema)]