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.

codex-rs/core/src/session_prefix.rs

@@ -1,28 +1,8 @@
 use codex_protocol::protocol::AgentStatus;
 
-/// Helpers for identifying model-visible "session prefix" messages.
-///
-/// A session prefix is a user-role message that carries configuration or state needed by
-/// follow-up turns (e.g. `<environment_context>`, `<turn_aborted>`). These items are persisted in
-/// history so the model can see them, but they are not user intent and must not create user-turn
-/// boundaries.
-pub(crate) const ENVIRONMENT_CONTEXT_OPEN_TAG: &str = "<environment_context>";
-pub(crate) const TURN_ABORTED_OPEN_TAG: &str = "<turn_aborted>";
-pub(crate) const SUBAGENT_NOTIFICATION_OPEN_TAG: &str = "<subagent_notification>";
-pub(crate) const SUBAGENT_NOTIFICATION_CLOSE_TAG: &str = "</subagent_notification>";
-
-fn starts_with_ascii_case_insensitive(text: &str, prefix: &str) -> bool {
-    text.get(..prefix.len())
-        .is_some_and(|candidate| candidate.eq_ignore_ascii_case(prefix))
-}
-
-/// Returns true if `text` starts with a session prefix marker (case-insensitive).
-pub(crate) fn is_session_prefix(text: &str) -> bool {
-    let trimmed = text.trim_start();
-    starts_with_ascii_case_insensitive(trimmed, ENVIRONMENT_CONTEXT_OPEN_TAG)
-        || starts_with_ascii_case_insensitive(trimmed, TURN_ABORTED_OPEN_TAG)
-        || starts_with_ascii_case_insensitive(trimmed, SUBAGENT_NOTIFICATION_OPEN_TAG)
-}
+/// Helpers for model-visible session state markers that are stored in user-role
+/// messages but are not user intent.
+use crate::contextual_user_message::SUBAGENT_NOTIFICATION_FRAGMENT;
 
 pub(crate) fn format_subagent_notification_message(agent_id: &str, status: &AgentStatus) -> String {
     let payload_json = serde_json::json!({
@@ -30,19 +10,5 @@ pub(crate) fn format_subagent_notification_message(agent_id: &str, status: &Agen
         "status": status,
     })
     .to_string();
-    format!("{SUBAGENT_NOTIFICATION_OPEN_TAG}\n{payload_json}\n{SUBAGENT_NOTIFICATION_CLOSE_TAG}")
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use pretty_assertions::assert_eq;
-
-    #[test]
-    fn is_session_prefix_is_case_insensitive() {
-        assert_eq!(
-            is_session_prefix("<SUBAGENT_NOTIFICATION>{}</subagent_notification>"),
-            true
-        );
-    }
+    SUBAGENT_NOTIFICATION_FRAGMENT.wrap(payload_json)
 }