chore: move memory prompt builder into extension (#24558)

## Why

The memories extension now owns the read-path developer instructions it
injects at thread start. Keeping that prompt builder and template in
`codex-memories-read` left the extension depending on a helper crate for
extension-specific prompt assembly, and kept async template/truncation
dependencies in the read crate after the remaining read surface no
longer needed them.

## What changed

- Moved `prompts.rs`, its tests, and `templates/memories/read_path.md`
from `memories/read` into `ext/memories`.
- Wired `MemoryExtension` to call the local prompt builder and added the
moved templates to `ext/memories/BUILD.bazel` compile data.
- Removed the now-unused prompt export and prompt-related dependencies
from `codex-memories-read`.

## Testing

- Not run locally.

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

@@ -10,10 +10,10 @@ use codex_extension_api::ThreadLifecycleContributor;
 use codex_extension_api::ThreadStartInput;
 use codex_extension_api::ToolContributor;
 use codex_features::Feature;
-use codex_memories_read::build_memory_tool_developer_instructions;
 use codex_utils_absolute_path::AbsolutePathBuf;
 
 use crate::local::LocalMemoriesBackend;
+use crate::prompts::build_memory_tool_developer_instructions;
 use crate::tools;
 
 /// Contributes Codex memory read-path prompt context and memory read tools.

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

@@ -1,6 +1,7 @@
 mod backend;
 mod extension;
 mod local;
+mod prompts;
 mod schema;
 mod tools;
 
@@ -11,6 +12,7 @@ pub(crate) const MAX_LIST_RESULTS: usize = 2_000;
 pub(crate) const DEFAULT_SEARCH_MAX_RESULTS: usize = 200;
 pub(crate) const MAX_SEARCH_RESULTS: usize = 200;
 pub(crate) const DEFAULT_READ_MAX_TOKENS: usize = 20_000;
+pub(crate) const MEMORY_TOOL_DEVELOPER_INSTRUCTIONS_SUMMARY_TOKEN_LIMIT: usize = 2_500;
 
 pub(crate) const MEMORY_TOOLS_NAMESPACE: &str = "memories/";
 pub(crate) const LIST_TOOL_NAME: &str = "list";

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

@@ -0,0 +1,55 @@
+use crate::MEMORY_TOOL_DEVELOPER_INSTRUCTIONS_SUMMARY_TOKEN_LIMIT;
+use codex_utils_absolute_path::AbsolutePathBuf;
+use codex_utils_output_truncation::TruncationPolicy;
+use codex_utils_output_truncation::truncate_text;
+use codex_utils_template::Template;
+use std::sync::LazyLock;
+use tokio::fs;
+
+static MEMORY_TOOL_DEVELOPER_INSTRUCTIONS_TEMPLATE: LazyLock<Template> = LazyLock::new(|| {
+    parse_embedded_template(
+        include_str!("../templates/memories/read_path.md"),
+        "memories/read_path.md",
+    )
+});
+
+fn parse_embedded_template(source: &'static str, template_name: &str) -> Template {
+    match Template::parse(source) {
+        Ok(template) => template,
+        Err(err) => panic!("embedded template {template_name} is invalid: {err}"),
+    }
+}
+
+/// Build the memory read-path prompt that is added to developer instructions.
+///
+/// Large `memory_summary.md` files are truncated at
+/// [MEMORY_TOOL_DEVELOPER_INSTRUCTIONS_SUMMARY_TOKEN_LIMIT].
+pub(crate) async fn build_memory_tool_developer_instructions(
+    codex_home: &AbsolutePathBuf,
+) -> Option<String> {
+    let base_path = codex_home.join("memories");
+    let memory_summary_path = base_path.join("memory_summary.md");
+    let memory_summary = fs::read_to_string(&memory_summary_path)
+        .await
+        .ok()?
+        .trim()
+        .to_string();
+    let memory_summary = truncate_text(
+        &memory_summary,
+        TruncationPolicy::Tokens(MEMORY_TOOL_DEVELOPER_INSTRUCTIONS_SUMMARY_TOKEN_LIMIT),
+    );
+    if memory_summary.is_empty() {
+        return None;
+    }
+    let base_path = base_path.display().to_string();
+    MEMORY_TOOL_DEVELOPER_INSTRUCTIONS_TEMPLATE
+        .render([
+            ("base_path", base_path.as_str()),
+            ("memory_summary", memory_summary.as_str()),
+        ])
+        .ok()
+}
+
+#[cfg(test)]
+#[path = "prompts_tests.rs"]
+mod tests;

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

@@ -0,0 +1,35 @@
+use super::*;
+use codex_utils_absolute_path::AbsolutePathBuf;
+use pretty_assertions::assert_eq;
+use tempfile::tempdir;
+use tokio::fs as tokio_fs;
+
+#[tokio::test]
+async fn build_memory_tool_developer_instructions_renders_embedded_template() {
+    let temp = tempdir().unwrap();
+    let codex_home = AbsolutePathBuf::from_absolute_path(temp.path()).unwrap();
+    let memories_dir = codex_home.join("memories");
+    tokio_fs::create_dir_all(&memories_dir).await.unwrap();
+    tokio_fs::write(
+        memories_dir.join("memory_summary.md"),
+        "Short memory summary for tests.",
+    )
+    .await
+    .unwrap();
+
+    let instructions = build_memory_tool_developer_instructions(&codex_home)
+        .await
+        .unwrap();
+
+    assert!(instructions.contains(&format!(
+        "- {}/memory_summary.md (already provided below; do NOT open again)",
+        memories_dir.display()
+    )));
+    assert!(instructions.contains("Short memory summary for tests."));
+    assert_eq!(
+        instructions
+            .matches("========= MEMORY_SUMMARY BEGINS =========")
+            .count(),
+        1
+    );
+}