Move host skill prompt injection into the skills extension (#37503)

## What changed

- Add `HostSkillsSnapshot::load_skill_prompts` so the skills extension owns reading and rendering selected host skills.
- Keep explicit invocation telemetry, unreadable-skill warnings, plugin prompt ordering, and suppression of host prompts superseded by provider prompts.
- Remove the legacy prompt injection and fragment implementation from `codex-core-skills`.

## Testing

- Cover prompt truncation without an installed skills extension, skill/plugin ordering, selective provider supersession, unreadable skills, and contextual fragment detection.

GitOrigin-RevId: 9c8b84ad54b90d174abc287ab49cc5a231e9e1aa

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

@@ -3,8 +3,6 @@ use std::collections::HashSet;
 use std::sync::Arc;
 use std::sync::atomic::Ordering;
 
-use crate::SkillInjections;
-use crate::build_skill_injections;
 use crate::client::ModelClientSession;
 use crate::client_common::Prompt;
 use crate::client_common::ResponseEvent;
@@ -24,9 +22,6 @@ use crate::hook_runtime::reject_pending_input;
 use crate::hook_runtime::run_legacy_after_agent_hook;
 use crate::hook_runtime::run_pending_session_start_hooks;
 use crate::hook_runtime::run_turn_stop_hooks;
-use crate::injection::ToolMentionKind;
-use crate::injection::app_id_from_path;
-use crate::injection::tool_kind_for_path;
 use crate::mcp_skill_dependencies::maybe_prompt_and_install_mcp_dependencies;
 use crate::mentions::build_connector_slug_counts;
 use crate::mentions::build_skill_name_counts;
@@ -44,6 +39,7 @@ use crate::session::TurnInput;
 use crate::session::session::Session;
 use crate::session::step_context::StepContext;
 use crate::session::turn_context::TurnContext;
+use crate::skills::emit_explicit_skill_invocations;
 use crate::stream_events_utils::HandleOutputCtx;
 use crate::stream_events_utils::TurnItemContributorPolicy;
 use crate::stream_events_utils::finalize_non_tool_response_item;
@@ -74,7 +70,6 @@ use codex_analytics::build_track_events_context;
 use codex_async_utils::OrCancelExt;
 use codex_connectors::AppToolPolicyEvaluator;
 use codex_core_plugins::RecommendedPluginCandidatesInput;
-use codex_core_skills::injection::InjectedHostSkillPrompts;
 use codex_extension_api::ExtensionData;
 use codex_extension_api::TurnInputContext;
 use codex_extension_api::TurnInputEnvironment;
@@ -111,6 +106,11 @@ use codex_protocol::protocol::SafetyBufferingEvent;
 use codex_protocol::protocol::TurnDiffEvent;
 use codex_protocol::protocol::WarningEvent;
 use codex_protocol::user_input::UserInput;
+use codex_skills::ToolMentionKind;
+use codex_skills::app_id_from_path;
+use codex_skills::tool_kind_for_path;
+use codex_skills_extension::HostSkillPrompts;
+use codex_skills_extension::InjectedHostSkillPrompts;
 use codex_tools::DiscoverableTool;
 use codex_tools::ToolName;
 use codex_tools::filter_request_plugin_install_discoverable_tools_for_client;
@@ -791,27 +791,26 @@ async fn build_skills_and_plugins(
     let injected_host_skill_prompts = turn_context
         .extension_data
         .get::<InjectedHostSkillPrompts>();
-    let SkillInjections {
-        items: skill_injections,
-        warnings: skill_warnings,
-    } = build_skill_injections(
+    let HostSkillPrompts {
+        fragments,
+        injected: injected_host_skills,
+        warnings: host_skill_warnings,
+    } = skills_snapshot.load_skill_prompts(&mentioned_skills).await;
+    emit_explicit_skill_invocations(
+        sess,
+        turn_context,
         &mentioned_skills,
-        Some(skills_outcome),
-        Some(&turn_context.session_telemetry),
-        &sess.services.analytics_events_client,
+        &injected_host_skills,
         tracking.clone(),
-    )
-    .await;
-
-    for message in skill_warnings {
+    );
+    for message in host_skill_warnings {
         sess.send_event(turn_context, EventMsg::Warning(WarningEvent { message }))
             .await;
     }
-
-    let skill_items: Vec<ResponseItem> = skill_injections
-        .iter()
-        .map(|skill| ContextualUserFragment::into(crate::context::SkillInstructions::from(skill)))
-        .collect();
+    let skill_items = fragments
+        .into_iter()
+        .map(ContextualUserFragment::into_boxed_response_item)
+        .collect::<Vec<_>>();
     let skill_connector_ids = collect_explicit_app_ids_from_skill_items(
         &skill_items,
         &available_connectors,
@@ -849,12 +848,14 @@ async fn build_skills_and_plugins(
         }
     }
 
-    let mut injection_items: Vec<ResponseItem> = match injected_host_skill_prompts {
-        Some(injected_host_skill_prompts) => skill_injections
-            .iter()
-            .filter(|skill| !injected_host_skill_prompts.contains_path(&skill.path))
-            .map(|skill| {
-                ContextualUserFragment::into(crate::context::SkillInstructions::from(skill))
+    let mut injection_items = match injected_host_skill_prompts {
+        Some(injected_host_skill_prompts) => skill_items
+            .into_iter()
+            .zip(injected_host_skills.iter())
+            .filter_map(|(item, skill)| {
+                (!injected_host_skill_prompts
+                    .contains_path(&skill.path_to_skills_md.to_string_lossy()))
+                .then_some(item)
             })
             .collect(),
         None => skill_items,