[codex] Load AGENTS.md from all bound environments (#27696)

## Why

We already have the machinery to support multiple environments on a
single thread, but we only show the model the contents of `AGENTS.md`
files in the primary environment.

We should show the model all of the relevant project instructions when
we know there's more than one environment.

## Known Gaps

As discussed in the RFC, this implementation:

1. doesn't handle environments being added/removed to/from the thread
after its creation
2. it doesn't enforce an aggregate context budget across environments,
and instead applies the configured project maximum independently to each
environment

## Implementation

- Discover project instructions in environment order with an independent
byte budget per environment and preserve source provenance/order.
- Keep the legacy fragment byte-for-byte when exactly one environment
contributes project instructions; use environment-labeled sections when
two or more environments contribute.
- Freeze the complete rendered fragment in `LoadedAgentsMd`, insert it
directly into requests, and recognize both layouts in contextual and
memory filtering.
- Add exact rendering, independent-budget, source-order,
creation-snapshot, and consumer coverage without changing app-server
schemas.

codex-rs/core/src/context/contextual_user_message_tests.rs

@@ -17,10 +17,38 @@ fn detects_environment_context_fragment() {
 
 #[test]
 fn detects_agents_instructions_fragment() {
-    assert!(is_contextual_user_fragment(&ContentItem::InputText {
-        text: "# AGENTS.md instructions for /tmp\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>"
-            .to_string(),
-    }));
+    for text in [
+        "# AGENTS.md instructions for /tmp\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>",
+        "# AGENTS.md instructions\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>",
+    ] {
+        assert!(is_contextual_user_fragment(&ContentItem::InputText {
+            text: text.to_string(),
+        }));
+    }
+}
+
+#[test]
+fn renders_agents_instructions_with_legacy_directory_header() {
+    assert_eq!(
+        UserInstructions {
+            directory: Some("/tmp".to_string()),
+            text: "body".to_string(),
+        }
+        .render(),
+        "# AGENTS.md instructions for /tmp\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>"
+    );
+}
+
+#[test]
+fn renders_agents_instructions_without_directory_header() {
+    assert_eq!(
+        UserInstructions {
+            directory: None,
+            text: "body".to_string(),
+        }
+        .render(),
+        "# AGENTS.md instructions\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>"
+    );
 }
 
 #[test]

codex-rs/core/src/context/user_instructions.rs

@@ -2,7 +2,7 @@ use super::ContextualUserFragment;
 
 #[derive(Debug, Clone, PartialEq)]
 pub(crate) struct UserInstructions {
-    pub(crate) directory: String,
+    pub(crate) directory: Option<String>,
     pub(crate) text: String,
 }
 
@@ -16,10 +16,15 @@ impl ContextualUserFragment for UserInstructions {
     }
 
     fn type_markers() -> (&'static str, &'static str) {
-        ("# AGENTS.md instructions for ", "</INSTRUCTIONS>")
+        ("# AGENTS.md instructions", "</INSTRUCTIONS>")
     }
 
     fn body(&self) -> String {
-        format!("{}\n\n<INSTRUCTIONS>\n{}\n", self.directory, self.text)
+        let directory = self
+            .directory
+            .as_ref()
+            .map(|directory| format!(" for {directory}"))
+            .unwrap_or_default();
+        format!("{directory}\n\n<INSTRUCTIONS>\n{}\n", self.text)
     }
 }