fix: policy/*.codexpolicy -> rules/*.rules (#7888)

We decided that `*.rules` is a more fitting (and concise) file extension
than `*.codexpolicy`, so we are changing the file extension for the
"execpolicy" effort. We are also changing the subfolder of `$CODEX_HOME`
from `policy` to `rules` to match.

This PR updates the in-repo docs and we will update the public docs once
the next CLI release goes out.

Locally, I created `~/.codex/rules/default.rules` with the following
contents:

```
prefix_rule(pattern=["gh", "pr", "view"])
```

And then I asked Codex to run:

```
gh pr view 7888 --json title,body,comments
```

and it was able to!

codex-rs/cli/tests/execpolicy.rs

@@ -8,7 +8,12 @@ use tempfile::TempDir;
 #[test]
 fn execpolicy_check_matches_expected_json() -> Result<(), Box<dyn std::error::Error>> {
     let codex_home = TempDir::new()?;
-    let policy_path = codex_home.path().join("policy.codexpolicy");
+    let policy_path = codex_home.path().join("rules").join("policy.rules");
+    fs::create_dir_all(
+        policy_path
+            .parent()
+            .expect("policy path should have a parent"),
+    )?;
     fs::write(
         &policy_path,
         r#"
@@ -24,7 +29,7 @@ prefix_rule(
         .args([
             "execpolicy",
             "check",
-            "--policy",
+            "--rules",
             policy_path
                 .to_str()
                 .expect("policy path should be valid UTF-8"),