core: bundle settings diff updates into one dev/user envelope (#12417)

## Summary
- bundle contextual prompt injection into at most one developer message
plus one contextual user message in both:
  - per-turn settings updates
  - initial context insertion
- preserve `<model_switch>` across compaction by rebuilding it through
canonical initial-context injection, instead of relying on
strip/reattach hacks
- centralize contextual user fragment detection in one shared definition
table and reuse it for parsing/compaction logic
- keep `AGENTS.md` in its natural serialized format:
  - `# AGENTS.md instructions for {dirname}`
  - `<INSTRUCTIONS>...</INSTRUCTIONS>`
- simplify related tests/helpers and accept the expected snapshot/layout
updates from bundled multi-part messages

## Why
The goal is to converge toward a simpler, more intentional prompt shape
where contextual updates are consistently represented as one developer
envelope plus one contextual user envelope, while keeping parsing and
compaction behavior aligned with that representation.

## Notable details
- the temporary `SettingsUpdateEnvelope` wrapper was removed; these
paths now return `Vec<ResponseItem>` directly
- local/remote compaction no longer rely on model-switch strip/restore
helpers
- contextual user detection is now driven by shared fragment definitions
instead of ad hoc matcher assembly
- AGENTS/user instructions are still the same logical context; only the
synthetic `<user_instructions>` wrapper was replaced by the natural
AGENTS text format

## Testing
- `just fmt`
- `cargo test -p codex-app-server
codex_message_processor::tests::extract_conversation_summary_prefers_plain_user_messages
-- --exact`
- `cargo test -p codex-core
compact::tests::collect_user_messages_filters_session_prefix_entries
--lib -- --exact`
- `cargo test -p codex-core --test all
'suite::compact::snapshot_request_shape_pre_turn_compaction_strips_incoming_model_switch'
-- --exact`
- `cargo test -p codex-core --test all
'suite::compact_remote::snapshot_request_shape_remote_pre_turn_compaction_strips_incoming_model_switch'
-- --exact`
- `cargo test -p codex-core --test all
'suite::client::includes_apps_guidance_as_developer_message_when_enabled'
-- --exact`
- `cargo test -p codex-core --test all
'suite::client::includes_developer_instructions_message_in_request' --
--exact`
- `cargo test -p codex-core --test all
'suite::client::includes_user_instructions_message_in_request' --
--exact`
- `cargo test -p codex-core --test all
'suite::client::resume_includes_initial_messages_and_sends_prior_items'
-- --exact`
- `cargo test -p codex-core --test all
'suite::review::review_input_isolated_from_parent_history' -- --exact`
- `cargo test -p codex-exec --test all
'suite::resume::exec_resume_last_respects_cwd_filter_and_all_flag' --
--exact`
- `cargo test -p core_test_support
context_snapshot::tests::full_text_mode_preserves_unredacted_text --
--exact`

## Notes
- I also ran several targeted `compact`, `compact_remote`,
`prompt_caching`, `model_visible_layout`, and `event_mapping` tests
while iterating on prompt-shape changes.
- I have not claimed a clean full-workspace `cargo test` from this
environment because local sandbox/resource conditions have previously
produced unrelated failures in large workspace runs.

Author: charley-oai

codex-rs/app-server/src/codex_message_processor.rs

@@ -7627,7 +7627,7 @@ mod tests {
                 "role": "user",
                 "content": [{
                     "type": "input_text",
-                    "text": "<user_instructions>\n<AGENTS.md contents>\n</user_instructions>".to_string(),
+                    "text": "# AGENTS.md instructions for project\n\n<INSTRUCTIONS>\n<AGENTS.md contents>\n</INSTRUCTIONS>".to_string(),
                 }],
             }),
             json!({

codex-rs/app-server/tests/suite/send_message.rs

@@ -214,17 +214,13 @@ async fn test_send_message_raw_notifications_opt_in() -> Result<()> {
         })
         .await?;
 
-    let permissions = read_raw_response_item(&mut mcp, conversation_id).await;
-    assert_permissions_message(&permissions);
-
     let developer = read_raw_response_item(&mut mcp, conversation_id).await;
+    assert_permissions_message(&developer);
     assert_developer_message(&developer, "Use the test harness tools.");
 
-    let instructions = read_raw_response_item(&mut mcp, conversation_id).await;
-    assert_instructions_message(&instructions);
-
-    let environment = read_raw_response_item(&mut mcp, conversation_id).await;
-    assert_environment_message(&environment);
+    let contextual_user = read_raw_response_item(&mut mcp, conversation_id).await;
+    assert_instructions_message(&contextual_user);
+    assert_environment_message(&contextual_user);
 
     let response: JSONRPCResponse = timeout(
         DEFAULT_READ_TIMEOUT,
@@ -545,9 +541,8 @@ fn assert_permissions_message(item: &ResponseItem) {
                 false,
             )
             .into_text();
-            assert_eq!(
-                texts,
-                vec![expected.as_str()],
+            assert!(
+                texts.iter().any(|text| *text == expected),
                 "expected permissions developer message, got {texts:?}"
             );
         }
@@ -560,9 +555,8 @@ fn assert_developer_message(item: &ResponseItem, expected_text: &str) {
         ResponseItem::Message { role, content, .. } => {
             assert_eq!(role, "developer");
             let texts = content_texts(content);
-            assert_eq!(
-                texts,
-                vec![expected_text],
+            assert!(
+                texts.contains(&expected_text),
                 "expected developer instructions message, got {texts:?}"
             );
         }

codex-rs/core/src/agent/control.rs

@@ -319,8 +319,8 @@ mod tests {
     use crate::config::Config;
     use crate::config::ConfigBuilder;
     use crate::config_loader::LoaderOverrides;
+    use crate::contextual_user_message::SUBAGENT_NOTIFICATION_OPEN_TAG;
     use crate::features::Feature;
-    use crate::session_prefix::SUBAGENT_NOTIFICATION_OPEN_TAG;
     use assert_matches::assert_matches;
     use codex_protocol::config_types::ModeKind;
     use codex_protocol::models::ContentItem;