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/event_mapping.rs

@@ -18,16 +18,15 @@ use codex_protocol::user_input::UserInput;
 use tracing::warn;
 use uuid::Uuid;
 
-use crate::instructions::SkillInstructions;
-use crate::instructions::UserInstructions;
-use crate::session_prefix::is_session_prefix;
-use crate::user_shell_command::is_user_shell_command_text;
+use crate::contextual_user_message::is_contextual_user_fragment;
 use crate::web_search::web_search_action_detail;
 
+pub(crate) fn is_contextual_user_message_content(message: &[ContentItem]) -> bool {
+    message.iter().any(is_contextual_user_fragment)
+}
+
 fn parse_user_message(message: &[ContentItem]) -> Option<UserMessageItem> {
-    if UserInstructions::is_user_instructions(message)
-        || SkillInstructions::is_skill_instructions(message)
-    {
+    if is_contextual_user_message_content(message) {
         return None;
     }
 
@@ -44,9 +43,6 @@ fn parse_user_message(message: &[ContentItem]) -> Option<UserMessageItem> {
                 {
                     continue;
                 }
-                if is_session_prefix(text) || is_user_shell_command_text(text) {
-                    return None;
-                }
                 content.push(UserInput::Text {
                     text: text.clone(),
                     // Model input content does not carry UI element ranges.
@@ -59,9 +55,6 @@ fn parse_user_message(message: &[ContentItem]) -> Option<UserMessageItem> {
                 });
             }
             ContentItem::OutputText { text } => {
-                if is_session_prefix(text) {
-                    return None;
-                }
                 warn!("Output text in user message: {}", text);
             }
         }
@@ -299,7 +292,7 @@ mod tests {
                 id: None,
                 role: "user".to_string(),
                 content: vec![ContentItem::InputText {
-                    text: "<user_instructions>test_text</user_instructions>".to_string(),
+                    text: "# AGENTS.md instructions for test_directory\n\n<INSTRUCTIONS>\ntest_text\n</INSTRUCTIONS>".to_string(),
                 }],
                 end_turn: None,
             phase: None,
@@ -341,6 +334,22 @@ mod tests {
                 end_turn: None,
             phase: None,
             },
+            ResponseItem::Message {
+                id: None,
+                role: "user".to_string(),
+                content: vec![
+                    ContentItem::InputText {
+                        text: "<environment_context>ctx</environment_context>".to_string(),
+                    },
+                    ContentItem::InputText {
+                        text:
+                            "# AGENTS.md instructions for dir\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>"
+                                .to_string(),
+                    },
+                ],
+                end_turn: None,
+                phase: None,
+            },
         ];
 
         for item in items {