fix: support $CODEX_HOME/AGENTS.md instead of $CODEX_HOME/instructions.md (#1891)

The docs and code do not match. It turns out the docs are "right" in
they are what we have been meaning to support, so this PR updates the
code:


ae88b69b09/README.md (L298-L302)

Support for `instructions.md` is a holdover from the TypeScript CLI, so
we are just going to drop support for it altogether rather than maintain
it in perpetuity.

codex-rs/core/src/config.rs

@@ -70,7 +70,7 @@ pub struct Config {
     /// who have opted into Zero Data Retention (ZDR).
     pub disable_response_storage: bool,
 
-    /// User-provided instructions from instructions.md.
+    /// User-provided instructions from AGENTS.md.
     pub user_instructions: Option<String>,
 
     /// Base instructions override.
@@ -575,7 +575,7 @@ impl Config {
             None => return None,
         };
 
-        p.push("instructions.md");
+        p.push("AGENTS.md");
         std::fs::read_to_string(&p).ok().and_then(|s| {
             let s = s.trim();
             if s.is_empty() {