[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/core/config.schema.json

@@ -810,7 +810,8 @@
     },
     "MarketplaceSourceType": {
       "enum": [
-        "git"
+        "git",
+        "local"
       ],
       "type": "string"
     },