Revert state DB injection and agent graph store (#21481)

## Why

Reverts #20689 to restore the previous optional state DB plumbing. The
conflict resolution keeps the newer installation ID and session/thread
identity changes that landed after #20689, while removing the mandatory
state DB and agent graph store dependency from ThreadManager
construction.

## What changed

- Restored `Option<StateDbHandle>` through app-server, MCP server,
prompt debug, and test entry points.
- Removed the `codex-core` dependency on `codex-agent-graph-store` and
reverted descendant lookup back to the existing state DB path when
available.
- Kept newer `installation_id` forwarding by passing it beside the
optional DB handle.
- Kept local thread-name updates working when the optional state DB
handle is absent.

## Validation

- `git diff --check`
- `cargo test -p codex-thread-store`
- `cargo test -p codex-state -p codex-rollout -p
codex-app-server-protocol`
- Attempted `env CARGO_INCREMENTAL=0 cargo test -p codex-core -p
codex-app-server -p codex-app-server-client -p codex-mcp-server -p
codex-thread-manager-sample -p codex-tui`; blocked locally by a rustc
ICE while compiling `v8 v146.4.0` with `rustc 1.93.0 (254b59607
2026-01-19)` on `aarch64-apple-darwin`.

codex-rs/thread-store/src/local/mod.rs

@@ -41,7 +41,7 @@ use crate::UpdateThreadMetadataParams;
 pub struct LocalThreadStore {
     pub(super) config: LocalThreadStoreConfig,
     live_recorders: Arc<Mutex<HashMap<ThreadId, RolloutRecorder>>>,
-    state_db: StateDbHandle,
+    state_db: Option<StateDbHandle>,
 }
 
 /// Process-scoped configuration for local thread storage.
@@ -51,6 +51,7 @@ pub struct LocalThreadStore {
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct LocalThreadStoreConfig {
     pub codex_home: PathBuf,
+    pub sqlite_home: PathBuf,
     /// Provider used only when older local metadata does not contain one.
     pub default_model_provider_id: String,
 }
@@ -59,6 +60,7 @@ impl LocalThreadStoreConfig {
     pub fn from_config(config: &impl codex_rollout::RolloutConfigView) -> Self {
         Self {
             codex_home: config.codex_home().to_path_buf(),
+            sqlite_home: config.sqlite_home().to_path_buf(),
             default_model_provider_id: config.model_provider_id().to_string(),
         }
     }
@@ -73,9 +75,8 @@ impl std::fmt::Debug for LocalThreadStore {
 }
 
 impl LocalThreadStore {
-    /// Create a local store from process-scoped local storage configuration and
-    /// the caller-provided shared state DB handle.
-    pub fn new(config: LocalThreadStoreConfig, state_db: StateDbHandle) -> Self {
+    /// Create a local store using an already initialized state DB handle.
+    pub fn new(config: LocalThreadStoreConfig, state_db: Option<StateDbHandle>) -> Self {
         Self {
             config,
             live_recorders: Arc::new(Mutex::new(HashMap::new())),
@@ -84,14 +85,10 @@ impl LocalThreadStore {
     }
 
     /// Return the state DB handle used by local rollout writers.
-    pub fn state_db(&self) -> StateDbHandle {
+    pub async fn state_db(&self) -> Option<StateDbHandle> {
         self.state_db.clone()
     }
 
-    pub(super) fn sqlite_home(&self) -> PathBuf {
-        self.state_db.codex_home().to_path_buf()
-    }
-
     /// Read a local rollout-backed thread by path.
     pub async fn read_thread_by_rollout_path(
         &self,
@@ -285,16 +282,14 @@ mod tests {
     use super::*;
     use crate::ThreadEventPersistenceMode;
     use crate::ThreadPersistenceMetadata;
-    use crate::local::test_support::init_test_state_db;
     use crate::local::test_support::test_config;
-    use crate::local::test_support::test_store;
     use crate::local::test_support::write_archived_session_file;
     use crate::local::test_support::write_session_file;
 
     #[tokio::test]
     async fn live_writer_lifecycle_writes_and_closes() {
         let home = TempDir::new().expect("temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let thread_id = ThreadId::default();
 
         store
@@ -343,7 +338,7 @@ mod tests {
     #[tokio::test]
     async fn create_thread_rejects_missing_cwd() {
         let home = TempDir::new().expect("temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let thread_id = ThreadId::default();
         let mut params = create_thread_params(thread_id);
         params.metadata.cwd = None;
@@ -363,7 +358,7 @@ mod tests {
     #[tokio::test]
     async fn discard_thread_drops_unmaterialized_live_writer() {
         let home = TempDir::new().expect("temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let thread_id = ThreadId::default();
 
         store
@@ -401,9 +396,8 @@ mod tests {
         let home = TempDir::new().expect("temp dir");
         let config = test_config(home.path());
         let thread_id = ThreadId::default();
-        let state_db = init_test_state_db(&config).await;
 
-        let first_store = LocalThreadStore::new(config.clone(), state_db.clone());
+        let first_store = LocalThreadStore::new(config.clone(), /*state_db*/ None);
         first_store
             .create_thread(create_thread_params(thread_id))
             .await
@@ -432,7 +426,7 @@ mod tests {
             .await
             .expect("shutdown initial writer");
 
-        let resumed_store = LocalThreadStore::new(config, state_db);
+        let resumed_store = LocalThreadStore::new(config, /*state_db*/ None);
         resumed_store
             .resume_thread(ResumeThreadParams {
                 thread_id,
@@ -463,7 +457,7 @@ mod tests {
     #[tokio::test]
     async fn create_thread_rejects_duplicate_live_writer() {
         let home = TempDir::new().expect("temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let thread_id = ThreadId::default();
 
         store
@@ -483,7 +477,7 @@ mod tests {
     #[tokio::test]
     async fn resume_thread_rejects_duplicate_live_writer() {
         let home = TempDir::new().expect("temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let thread_id = ThreadId::default();
 
         store
@@ -512,7 +506,7 @@ mod tests {
     #[tokio::test]
     async fn resume_thread_rejects_missing_cwd() {
         let home = TempDir::new().expect("temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let uuid = uuid::Uuid::from_u128(407);
         let thread_id = ThreadId::from_string(&uuid.to_string()).expect("valid thread id");
         let rollout_path =
@@ -541,7 +535,7 @@ mod tests {
     async fn load_history_uses_live_writer_rollout_path() {
         let home = TempDir::new().expect("temp dir");
         let external_home = TempDir::new().expect("external temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let uuid = uuid::Uuid::from_u128(404);
         let thread_id = ThreadId::from_string(&uuid.to_string()).expect("valid thread id");
         let rollout_path = write_session_file(external_home.path(), "2025-01-04T10-00-00", uuid)
@@ -590,7 +584,7 @@ mod tests {
     async fn read_thread_uses_live_writer_rollout_path_for_external_resume() {
         let home = TempDir::new().expect("temp dir");
         let external_home = TempDir::new().expect("external temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let uuid = uuid::Uuid::from_u128(406);
         let thread_id = ThreadId::from_string(&uuid.to_string()).expect("valid thread id");
         let rollout_path = write_session_file(external_home.path(), "2025-01-04T11-00-00", uuid)
@@ -629,7 +623,7 @@ mod tests {
     #[tokio::test]
     async fn load_history_uses_live_writer_rollout_path_for_archived_source() {
         let home = TempDir::new().expect("temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let uuid = uuid::Uuid::from_u128(405);
         let thread_id = ThreadId::from_string(&uuid.to_string()).expect("valid thread id");
         let rollout_path = write_archived_session_file(home.path(), "2025-01-04T10-30-00", uuid)
@@ -697,7 +691,7 @@ mod tests {
     #[tokio::test]
     async fn read_thread_by_rollout_path_includes_history() {
         let home = TempDir::new().expect("temp dir");
-        let store = test_store(home.path()).await;
+        let store = LocalThreadStore::new(test_config(home.path()), /*state_db*/ None);
         let thread_id = ThreadId::default();
 
         store