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/core-api/src/lib.rs

@@ -31,7 +31,6 @@ pub use codex_core::StartThreadOptions;
 pub use codex_core::StateDbHandle;
 pub use codex_core::ThreadManager;
 pub use codex_core::ThreadShutdownReport;
-pub use codex_core::agent_graph_store_from_state_db;
 pub use codex_core::config::Config;
 pub use codex_core::config::Constrained;
 pub use codex_core::config::GhostSnapshotConfig;
@@ -41,7 +40,6 @@ pub use codex_core::config::TerminalResizeReflowConfig;
 pub use codex_core::config::ThreadStoreConfig;
 pub use codex_core::config::find_codex_home;
 pub use codex_core::init_state_db;
-pub use codex_core::init_state_db_from_config;
 pub use codex_core::resolve_installation_id;
 pub use codex_core::skills::SkillsManager;
 pub use codex_core::thread_store_from_config;