Organize context fragments (#18794)

Organize context fragments under `core/context`. Implement same trait on
all of them.

codex-rs/core/src/session/mod.rs

@@ -19,6 +19,7 @@ use crate::commit_attribution::commit_message_trailer_instruction;
 use crate::compact;
 use crate::config::ManagedFeatures;
 use crate::connectors;
+use crate::context::ContextualUserFragment;
 use crate::default_skill_metadata_budget;
 use crate::exec_policy::ExecPolicyManager;
 use crate::installation_id::resolve_installation_id;
@@ -153,7 +154,6 @@ use crate::config::StartedNetworkProxy;
 use crate::config::resolve_web_search_mode_for_turn;
 use crate::context_manager::ContextManager;
 use crate::context_manager::TotalTokenUsageBreakdown;
-use crate::environment_context::EnvironmentContext;
 use crate::thread_rollout_truncation::initial_history_has_prior_user_turns;
 use codex_config::CONFIG_TOML_FILE;
 use codex_config::types::McpServerConfig;
@@ -249,9 +249,9 @@ use crate::SkillLoadOutcome;
 use crate::SkillMetadata;
 use crate::SkillsManager;
 use crate::agents_md::AgentsMdManager;
+use crate::context::UserInstructions;
 use crate::exec_policy::ExecPolicyUpdateError;
 use crate::guardian::GuardianReviewSessionManager;
-use crate::instructions::UserInstructions;
 use crate::mcp::McpManager;
 use crate::memories;
 use crate::network_policy_decision::execpolicy_network_rule_amendment;
@@ -2487,7 +2487,7 @@ impl Session {
                     text: user_instructions.to_string(),
                     directory: turn_context.cwd.to_string_lossy().into_owned(),
                 }
-                .serialize_to_text(),
+                .render(),
             );
         }
         if turn_context.config.include_environment_context {
@@ -2497,9 +2497,9 @@ impl Session {
                 .format_environment_context_subagents(self.conversation_id)
                 .await;
             contextual_user_sections.push(
-                EnvironmentContext::from_turn_context(turn_context, shell.as_ref())
+                crate::context::EnvironmentContext::from_turn_context(turn_context, shell.as_ref())
                     .with_subagents(subagents)
-                    .serialize_to_xml(),
+                    .render(),
             );
         }
 

codex-rs/core/src/session/tests.rs

@@ -11,6 +11,8 @@ use crate::config_loader::NetworkDomainPermissionsToml;
 use crate::config_loader::RequirementSource;
 use crate::config_loader::Sourced;
 use crate::config_loader::project_trust_key;
+use crate::context::ContextualUserFragment;
+use crate::context::TurnAborted;
 use crate::exec::ExecCapturePolicy;
 use crate::function_tool::FunctionCallError;
 use crate::shell::default_user_shell;
@@ -6349,7 +6351,7 @@ async fn abort_review_task_emits_exited_then_aborted_and_records_history() {
                 let ContentItem::InputText { text } = content_item else {
                     return false;
                 };
-                text.contains(crate::contextual_user_message::TURN_ABORTED_OPEN_TAG)
+                TurnAborted::matches_text(text)
             })
         }),
         "expected a model-visible turn aborted marker in history after interrupt"

codex-rs/core/src/session/turn.rs

@@ -16,6 +16,7 @@ use crate::compact::run_inline_auto_compact_task;
 use crate::compact::should_use_remote_compact_task;
 use crate::compact_remote::run_inline_remote_auto_compact_task;
 use crate::connectors;
+use crate::context::ContextualUserFragment;
 use crate::feedback_tags;
 use crate::hook_runtime::PendingInputHookDisposition;
 use crate::hook_runtime::emit_hook_completed_events;
@@ -241,7 +242,7 @@ pub(crate) async fn run_turn(
         turn_context.sub_id.clone(),
     );
     let SkillInjections {
-        items: skill_items,
+        items: skill_injections,
         warnings: skill_warnings,
     } = build_skill_injections(
         &mentioned_skills,
@@ -257,6 +258,11 @@ pub(crate) async fn run_turn(
             .await;
     }
 
+    let skill_items: Vec<ResponseItem> = skill_injections
+        .iter()
+        .map(|skill| ContextualUserFragment::into(crate::context::SkillInstructions::from(skill)))
+        .collect();
+
     let plugin_items =
         build_plugin_injections(&mentioned_plugins, &mcp_tools, &available_connectors);
     let mentioned_plugin_metadata = mentioned_plugins