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/config/src/marketplace_edit.rs

@@ -12,6 +12,7 @@ use crate::CONFIG_TOML_FILE;
 
 pub struct MarketplaceConfigUpdate<'a> {
     pub last_updated: &'a str,
+    pub last_revision: Option<&'a str>,
     pub source_type: &'a str,
     pub source: &'a str,
     pub ref_name: Option<&'a str>,
@@ -63,6 +64,9 @@ fn upsert_marketplace(
     let mut entry = TomlTable::new();
     entry.set_implicit(false);
     entry["last_updated"] = value(update.last_updated.to_string());
+    if let Some(last_revision) = update.last_revision {
+        entry["last_revision"] = value(last_revision.to_string());
+    }
     entry["source_type"] = value(update.source_type.to_string());
     entry["source"] = value(update.source.to_string());
     if let Some(ref_name) = update.ref_name {

codex-rs/config/src/types.rs

@@ -626,6 +626,9 @@ pub struct MarketplaceConfig {
     /// Last time Codex successfully added or refreshed this marketplace.
     #[serde(default)]
     pub last_updated: Option<String>,
+    /// Git revision Codex last successfully activated for this marketplace.
+    #[serde(default)]
+    pub last_revision: Option<String>,
     /// Source kind used to install this marketplace.
     #[serde(default)]
     pub source_type: Option<MarketplaceSourceType>,