Move memory prompt injection to app-server extension (#22841)

## Why

Memory prompt injection should be owned by the extension path that
app-server composes at runtime, not by an inlined special case inside
`codex-core`. This keeps `codex-core` focused on session orchestration
while allowing the memories extension to own its app-server prompt
behavior.

## What Changed

- Registers `codex-memories-extension` in the app-server extension
registry.
- Moves the memory developer-instruction injection out of
`core/src/session/mod.rs` and into the memories extension prompt
contributor.
- Adds config-change handling so the extension keeps its per-thread
memory settings in sync after startup.
- Leaves memories read/retrieval tools unregistered for now so this PR
only changes prompt injection.
- Removes the stale `cargo-shear` ignore now that app-server depends on
the extension crate.

## Validation

Not run locally; validation is left to CI.

codex-rs/Cargo.lock

@@ -1907,6 +1907,7 @@ dependencies = [
  "codex-hooks",
  "codex-login",
  "codex-mcp",
+ "codex-memories-extension",
  "codex-memories-write",
  "codex-model-provider",
  "codex-model-provider-info",

codex-rs/Cargo.toml

@@ -470,7 +470,6 @@ unwrap_used = "deny"
 [workspace.metadata.cargo-shear]
 ignored = [
     "codex-agent-graph-store",
-    "codex-memories-extension",
     "icu_provider",
     "openssl-sys",
     "codex-v8-poc",

codex-rs/app-server/Cargo.toml

@@ -54,6 +54,7 @@ codex-backend-client = { workspace = true }
 codex-file-search = { workspace = true }
 codex-chatgpt = { workspace = true }
 codex-login = { workspace = true }
+codex-memories-extension = { workspace = true }
 codex-memories-write = { workspace = true }
 codex-mcp = { workspace = true }
 codex-model-provider = { workspace = true }

codex-rs/app-server/src/extensions.rs

@@ -18,6 +18,7 @@ where
 {
     let mut builder = ExtensionRegistryBuilder::<Config>::new();
     codex_guardian::install(&mut builder, guardian_agent_spawner);
+    codex_memories_extension::install(&mut builder);
     Arc::new(builder.build())
 }
 

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

@@ -2690,14 +2690,6 @@ impl Session {
         {
             developer_sections.push(developer_instructions.to_string());
         }
-        // Add developer instructions for memories.
-        if turn_context.features.enabled(Feature::MemoryTool)
-            && turn_context.config.memories.use_memories
-            && let Some(memory_prompt) =
-                build_memory_tool_developer_instructions(&turn_context.config.codex_home).await
-        {
-            developer_sections.push(memory_prompt);
-        }
         // Add developer instructions from collaboration_mode if they exist and are non-empty
         if turn_context.config.include_collaboration_mode_instructions
             && let Some(collab_instructions) =
@@ -3438,8 +3430,6 @@ pub(crate) fn emit_subagent_session_started(
     });
 }
 
-use codex_memories_read::build_memory_tool_developer_instructions;
-
 /// Builds the hook engine for one config snapshot, including any enabled plugin hooks.
 async fn build_hooks_for_config(
     config: &Config,

codex-rs/ext/memories/src/extension.rs

@@ -1,6 +1,7 @@
 use std::sync::Arc;
 
 use codex_core::config::Config;
+use codex_extension_api::ConfigContributor;
 use codex_extension_api::ContextContributor;
 use codex_extension_api::ExtensionData;
 use codex_extension_api::ExtensionRegistryBuilder;
@@ -25,6 +26,15 @@ pub(crate) struct MemoriesExtensionConfig {
     pub(crate) codex_home: AbsolutePathBuf,
 }
 
+impl MemoriesExtensionConfig {
+    fn from_config(config: &Config) -> Self {
+        Self {
+            enabled: config.features.enabled(Feature::MemoryTool) && config.memories.use_memories,
+            codex_home: config.codex_home.clone(),
+        }
+    }
+}
+
 impl ContextContributor for MemoriesExtension {
     fn contribute<'a>(
         &'a self,
@@ -50,11 +60,21 @@ impl ContextContributor for MemoriesExtension {
 
 impl ThreadLifecycleContributor<Config> for MemoriesExtension {
     fn on_thread_start(&self, input: ThreadStartInput<'_, Config>) {
-        input.thread_store.insert(MemoriesExtensionConfig {
+        input
-            enabled: input.config.features.enabled(Feature::MemoryTool)
+            .thread_store
-                && input.config.memories.use_memories,
+            .insert(MemoriesExtensionConfig::from_config(input.config));
-            codex_home: input.config.codex_home.clone(),
+    }
-        });
+}
+
+impl ConfigContributor<Config> for MemoriesExtension {
+    fn on_config_changed(
+        &self,
+        _session_store: &ExtensionData,
+        thread_store: &ExtensionData,
+        _previous_config: &Config,
+        new_config: &Config,
+    ) {
+        thread_store.insert(MemoriesExtensionConfig::from_config(new_config));
     }
 }
 
@@ -79,6 +99,8 @@ impl ToolContributor for MemoriesExtension {
 pub fn install(registry: &mut ExtensionRegistryBuilder<Config>) {
     let extension = Arc::new(MemoriesExtension);
     registry.thread_lifecycle_contributor(extension.clone());
-    registry.prompt_contributor(extension.clone());
+    registry.config_contributor(extension.clone());
-    registry.tool_contributor(extension);
+    registry.prompt_contributor(extension);
+    // Keep the read/retrieval tools out of app-server until that rollout is intentional.
+    // registry.tool_contributor(extension);
 }