chore(core) Create instructions module (#9422)

## Summary
We have a variety of things we refer to as instructions in the code
base: our current canonical terms are:
- base instructions (raw string)
- developer instructions (has a type in protocol)
- user instructions

We also have `instructions` floating around in various places. We should
standardize on the above, and start using types to prevent them from
ending up in the wrong place. There will be additional PRs, but I'm
going to keep these small so we can easily follow them!

## Testing
- [x] Tests pass, this is purely a file move

codex-rs/core/src/codex.rs

@@ -98,6 +98,7 @@ use crate::error::Result as CodexResult;
 use crate::exec::StreamOutput;
 use crate::exec_policy::ExecPolicyUpdateError;
 use crate::feedback_tags;
+use crate::instructions::UserInstructions;
 use crate::mcp::auth::compute_auth_statuses;
 use crate::mcp_connection_manager::McpConnectionManager;
 use crate::model_provider_info::CHAT_WIRE_API_DEPRECATION_SUMMARY;
@@ -155,7 +156,6 @@ use crate::tools::spec::ToolsConfig;
 use crate::tools::spec::ToolsConfigParams;
 use crate::turn_diff_tracker::TurnDiffTracker;
 use crate::unified_exec::UnifiedExecProcessManager;
-use crate::user_instructions::UserInstructions;
 use crate::user_notification::UserNotification;
 use crate::util::backoff;
 use codex_async_utils::OrCancelExt;

codex-rs/core/src/context_manager/history.rs

@@ -1,12 +1,12 @@
 use crate::codex::TurnContext;
 use crate::context_manager::normalize;
+use crate::instructions::SkillInstructions;
+use crate::instructions::UserInstructions;
 use crate::truncate::TruncationPolicy;
 use crate::truncate::approx_token_count;
 use crate::truncate::approx_tokens_from_byte_count;
 use crate::truncate::truncate_function_output_items_with_policy;
 use crate::truncate::truncate_text;
-use crate::user_instructions::SkillInstructions;
-use crate::user_instructions::UserInstructions;
 use crate::user_shell_command::is_user_shell_command_text;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::FunctionCallOutputContentItem;

codex-rs/core/src/event_mapping.rs

@@ -17,8 +17,8 @@ use codex_protocol::user_input::UserInput;
 use tracing::warn;
 use uuid::Uuid;
 
-use crate::user_instructions::SkillInstructions;
-use crate::user_instructions::UserInstructions;
+use crate::instructions::SkillInstructions;
+use crate::instructions::UserInstructions;
 use crate::user_shell_command::is_user_shell_command_text;
 
 fn is_session_prefix(text: &str) -> bool {

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

@@ -0,0 +1,6 @@
+mod user_instructions;
+
+pub(crate) use user_instructions::SkillInstructions;
+pub use user_instructions::USER_INSTRUCTIONS_OPEN_TAG_LEGACY;
+pub use user_instructions::USER_INSTRUCTIONS_PREFIX;
+pub(crate) use user_instructions::UserInstructions;

codex-rs/core/src/lib.rs

@@ -31,6 +31,7 @@ mod exec_policy;
 pub mod features;
 mod flags;
 pub mod git_info;
+pub mod instructions;
 pub mod landlock;
 pub mod mcp;
 mod mcp_connection_manager;
@@ -50,7 +51,6 @@ mod text_encoding;
 pub mod token_data;
 mod truncate;
 mod unified_exec;
-mod user_instructions;
 pub mod windows_sandbox;
 pub use model_provider_info::CHAT_WIRE_API_DEPRECATION_SUMMARY;
 pub use model_provider_info::DEFAULT_LMSTUDIO_PORT;

codex-rs/core/src/skills/injection.rs

@@ -1,9 +1,9 @@
 use std::collections::HashSet;
 use std::path::PathBuf;
 
+use crate::instructions::SkillInstructions;
 use crate::skills::SkillLoadOutcome;
 use crate::skills::SkillMetadata;
-use crate::user_instructions::SkillInstructions;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::user_input::UserInput;
 use tokio::fs;