[codex] Add cross-repo plugin sources to marketplace manifests (#18017)

## Summary
- add first-class marketplace support for git-backed plugin sources
- keep the newer marketplace parsing behavior from `main`, including
alternate manifest locations and string local sources
- materialize remote plugin sources during install, detail reads, and
non-curated cache refresh
- expose git plugin source metadata through the app-server protocol

## Details
This teaches the marketplace parser to accept all of the following:
- local string sources such as `"source": "./plugins/foo"`
- local object sources such as
`{"source":"local","path":"./plugins/foo"}`
- remote repo-root sources such as
`{"source":"url","url":"https://github.com/org/repo.git"}`
- remote subdir sources such as
`{"source":"git-subdir","url":"owner/repo","path":"plugins/foo","ref":"main","sha":"..."}`

It also preserves the newer tolerant behavior from `main`: invalid or
unsupported plugin entries are skipped instead of breaking the whole
marketplace.

## Validation
- `cargo test -p codex-core plugins::marketplace::tests`
- `just fix -p codex-core`
- `just fmt`

## Notes
- A full `cargo test -p codex-core` run still hit unrelated existing
failures in agent and multi-agent tests during this session; the
marketplace-focused suite passed after the rebase resolution.

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -3707,6 +3707,14 @@ pub enum PluginSource {
     #[serde(rename_all = "camelCase")]
     #[ts(rename_all = "camelCase")]
     Local { path: AbsolutePathBuf },
+    #[serde(rename_all = "camelCase")]
+    #[ts(rename_all = "camelCase")]
+    Git {
+        url: String,
+        path: Option<String>,
+        ref_name: Option<String>,
+        sha: Option<String>,
+    },
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]