feat: add support for AGENTS.md in Rust CLI (#885)

The TypeScript CLI already has support for including the contents of
`AGENTS.md` in the instructions sent with the first turn of a
conversation. This PR brings this functionality to the Rust CLI.

To be considered, `AGENTS.md` must be in the `cwd` of the session, or in
one of the parent folders up to a Git/filesystem root (whichever is
encountered first).

By default, a maximum of 32 KiB of `AGENTS.md` will be included, though
this is configurable using the new-in-this-PR `project_doc_max_bytes`
option in `config.toml`.

codex-rs/core/src/codex.rs

@@ -52,6 +52,7 @@ use crate::models::FunctionCallOutputPayload;
 use crate::models::ResponseInputItem;
 use crate::models::ResponseItem;
 use crate::models::ShellToolCallParams;
+use crate::project_doc::create_full_instructions;
 use crate::protocol::AskForApproval;
 use crate::protocol::Event;
 use crate::protocol::EventMsg;
@@ -83,10 +84,12 @@ impl Codex {
     pub async fn spawn(config: Config, ctrl_c: Arc<Notify>) -> CodexResult<(Codex, String)> {
         let (tx_sub, rx_sub) = async_channel::bounded(64);
         let (tx_event, rx_event) = async_channel::bounded(64);
+
+        let instructions = create_full_instructions(&config).await;
         let configure_session = Op::ConfigureSession {
             provider: config.model_provider.clone(),
             model: config.model.clone(),
-            instructions: config.instructions.clone(),
+            instructions,
             approval_policy: config.approval_policy,
             sandbox_policy: config.sandbox_policy.clone(),
             disable_response_storage: config.disable_response_storage,