Cap project instructions across environments (#37424)

## Why

Applying `project_doc_max_bytes` independently to every selected environment
allows the total project instruction payload to grow with the number of
environments.

## What changed

Treat `project_doc_max_bytes` as one shared byte budget. Consume it in selected
environment order, truncate the entry that reaches the remaining limit, and
skip later environments once the budget is exhausted.

Update the configuration descriptions to document the aggregate limit.

## Testing

Add unit and integration coverage for partial and exhausted budgets across
multiple environments.

GitOrigin-RevId: 4664bdf1f23b66986349d503f44cf51560b9024f

codex-rs/core/src/agents_md.rs

@@ -55,17 +55,28 @@ pub(crate) async fn load_project_instructions(
     environments: &TurnEnvironmentSnapshot,
 ) -> Option<LoadedAgentsMd> {
     let mut loaded = LoadedAgentsMd::from_user_instructions(user_instructions);
+    let mut remaining = config.project_doc_max_bytes;
     for turn_environment in environments.turn_environments() {
+        if remaining == 0 {
+            break;
+        }
+
         let filesystem = turn_environment.environment.get_filesystem();
         match read_agents_md(
             config,
             filesystem.as_ref(),
             &turn_environment.environment_id,
             turn_environment.cwd(),
+            remaining,
         )
         .await
         {
-            Ok(Some(docs)) => loaded.entries.extend(docs.entries),
+            Ok(Some(docs)) => {
+                for entry in docs.entries {
+                    remaining = remaining.saturating_sub(entry.contents.len());
+                    loaded.entries.push(entry);
+                }
+            }
             Ok(None) => {}
             Err(e) => {
                 error!(
@@ -90,9 +101,8 @@ async fn read_agents_md(
     fs: &dyn ExecutorFileSystem,
     environment_id: &str,
     cwd: &PathUri,
+    max_total: usize,
 ) -> io::Result<Option<LoadedAgentsMd>> {
-    let max_total = config.project_doc_max_bytes;
-
     if max_total == 0 {
         return Ok(None);
     }