make codex better at git (#10145)

adds basic git context to the session prefix so the model can anchor git
actions and be a bit more version-aware. structured it in a
multiroot-friendly shape even though we only have one root today

codex-rs/core/src/turn_metadata.rs

@@ -0,0 +1,43 @@
+use std::collections::BTreeMap;
+use std::path::Path;
+
+use serde::Serialize;
+
+use crate::git_info::get_git_remote_urls_assume_git_repo;
+use crate::git_info::get_git_repo_root;
+use crate::git_info::get_head_commit_hash;
+
+#[derive(Serialize)]
+struct TurnMetadataWorkspace {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    associated_remote_urls: Option<BTreeMap<String, String>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    latest_git_commit_hash: Option<String>,
+}
+
+#[derive(Serialize)]
+struct TurnMetadata {
+    workspaces: BTreeMap<String, TurnMetadataWorkspace>,
+}
+
+pub(crate) async fn build_turn_metadata_header(cwd: &Path) -> Option<String> {
+    let repo_root = get_git_repo_root(cwd)?;
+
+    let (latest_git_commit_hash, associated_remote_urls) = tokio::join!(
+        get_head_commit_hash(cwd),
+        get_git_remote_urls_assume_git_repo(cwd)
+    );
+    if latest_git_commit_hash.is_none() && associated_remote_urls.is_none() {
+        return None;
+    }
+
+    let mut workspaces = BTreeMap::new();
+    workspaces.insert(
+        repo_root.to_string_lossy().into_owned(),
+        TurnMetadataWorkspace {
+            associated_remote_urls,
+            latest_git_commit_hash,
+        },
+    );
+    serde_json::to_string(&TurnMetadata { workspaces }).ok()
+}