## 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.
## Summary
Move `codex marketplace add` onto a shared core implementation so the
CLI and app-server path can use one source of truth.
This change:
- adds shared marketplace-add orchestration in `codex-core`
- switches the CLI command to call that shared implementation
- removes duplicated CLI-only marketplace add helpers
- preserves focused parser and add-path coverage while moving the shared
behavior into core tests
## Why
The new `marketplace/add` RPC should reuse the same underlying
marketplace-add flow as the CLI. This refactor lands that consolidation
first so the follow-up app-server PR can be mostly protocol and handler
wiring.
## Validation
- `cargo test -p codex-core marketplace_add`
- `cargo test -p codex-cli marketplace_cmd`
- `just fix -p codex-core`
- `just fix -p codex-cli`
- `just fmt`
