fix: agent instructions were not being included when ~/.codex/instructions.md was empty (#908)

I had seen issues where `codex-rs` would not always write files without
me pressuring it to do so, and between that and the report of
https://github.com/openai/codex/issues/900, I decided to look into this
further. I found two serious issues with agent instructions:

(1) We were only sending agent instructions on the first turn, but
looking at the TypeScript code, we should be sending them on every turn.

(2) There was a serious issue where the agent instructions were
frequently lost:

* The TypeScript CLI appears to keep writing `~/.codex/instructions.md`:
55142e3e6c/codex-cli/src/utils/config.ts (L586)
* If `instructions.md` is present, the Rust CLI uses the contents of it
INSTEAD OF the default prompt, even if `instructions.md` is empty:
55142e3e6c/codex-rs/core/src/config.rs (L202-L203)

The combination of these two things means that I have been using
`codex-rs` without these key instructions:
https://github.com/openai/codex/blob/main/codex-rs/core/prompt.md

Looking at the TypeScript code, it appears we should be concatenating
these three items every time (if they exist):

* `prompt.md`
* `~/.codex/instructions.md`
* nearest `AGENTS.md`

This PR fixes things so that:

* `Config.instructions` is `None` if `instructions.md` is empty
* `Payload.instructions` is now `&'a str` instead of `Option<&'a
String>` because we should always have _something_ to send
* `Prompt` now has a `get_full_instructions()` helper that returns a
`Cow<str>` that will always include the agent instructions first.

codex-rs/core/src/config.rs

@@ -10,11 +10,6 @@ use serde::Deserialize;
 use std::collections::HashMap;
 use std::path::PathBuf;
 
-/// Embedded fallback instructions that mirror the TypeScript CLI’s default
-/// system prompt. These are compiled into the binary so a clean install behaves
-/// correctly even if the user has not created `~/.codex/instructions.md`.
-const EMBEDDED_INSTRUCTIONS: &str = include_str!("../prompt.md");
-
 /// Maximum number of bytes of the documentation that will be embedded. Larger
 /// files are *silently truncated* to this size so we do not take up too much of
 /// the context window.
@@ -42,7 +37,7 @@ pub struct Config {
     /// who have opted into Zero Data Retention (ZDR).
     pub disable_response_storage: bool,
 
-    /// System instructions.
+    /// User-provided instructions from instructions.md.
     pub instructions: Option<String>,
 
     /// Optional external notifier command. When set, Codex will spawn this
@@ -198,9 +193,7 @@ impl Config {
         cfg: ConfigToml,
         overrides: ConfigOverrides,
     ) -> std::io::Result<Self> {
-        // Instructions: user-provided instructions.md > embedded default.
-        let instructions =
-            Self::load_instructions().or_else(|| Some(EMBEDDED_INSTRUCTIONS.to_string()));
+        let instructions = Self::load_instructions();
 
         // Destructure ConfigOverrides fully to ensure all overrides are applied.
         let ConfigOverrides {
@@ -289,7 +282,14 @@ impl Config {
     fn load_instructions() -> Option<String> {
         let mut p = codex_dir().ok()?;
         p.push("instructions.md");
-        std::fs::read_to_string(&p).ok()
+        std::fs::read_to_string(&p).ok().and_then(|s| {
+            let s = s.trim();
+            if s.is_empty() {
+                None
+            } else {
+                Some(s.to_string())
+            }
+        })
     }
 
     /// Meant to be used exclusively for tests: `load_with_overrides()` should