Auto-upgrade configured marketplaces (#17425)

## Summary
- Add best-effort auto-upgrade for user-configured Git marketplaces
recorded in `config.toml`.
- Track the last activated Git revision with `last_revision` so
unchanged marketplace sources skip clone work.
- Trigger the upgrade from plugin startup and `plugin/list`, while
preserving existing fail-open plugin behavior with warning logs rather
than new user-visible errors.

## Details
- Remote configured marketplaces use `git ls-remote` to compare the
source/ref against the recorded revision.
- Upgrades clone into a staging directory, validate that
`.agents/plugins/marketplace.json` exists and that the manifest name
matches the configured marketplace key, then atomically activate the new
root.
- Local `.agents/plugins/marketplace.json` marketplaces remain live
filesystem state and are not auto-pulled.
- Existing non-curated plugin cache refresh is kicked after successful
marketplace root upgrades.

## Validation
- `just write-config-schema`
- `cargo test -p codex-core marketplace_upgrade`
- `cargo check -p codex-cli -p codex-app-server`
- `just fix -p codex-core`

Did not run the complete `cargo test` suite because the repo
instructions require asking before a full core workspace run.

codex-rs/core/src/plugins/mod.rs

@@ -11,6 +11,8 @@ mod startup_sync;
 #[cfg(test)]
 pub(crate) mod test_support;
 
+pub use codex_core_plugins::marketplace_upgrade::ConfiguredMarketplaceUpgradeError as PluginMarketplaceUpgradeError;
+pub use codex_core_plugins::marketplace_upgrade::ConfiguredMarketplaceUpgradeOutcome as PluginMarketplaceUpgradeOutcome;
 pub use codex_plugin::AppConnectorId;
 pub use codex_plugin::EffectiveSkillRoots;
 pub use codex_plugin::PluginCapabilitySummary;