[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/cli/src/marketplace_cmd.rs

@@ -23,7 +23,8 @@ enum MarketplaceSubcommand {
 
 #[derive(Debug, Parser)]
 struct AddMarketplaceArgs {
-    /// Marketplace source. Supports owner/repo[@ref], HTTP(S) Git URLs, or SSH URLs.
+    /// Marketplace source. Supports owner/repo[@ref], HTTP(S) Git URLs, SSH URLs,
+    /// or local marketplace root directories.
     source: String,
 
     /// Git ref to check out. Overrides any @ref or #ref suffix in SOURCE.