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/exec-server/tests/common/lib.rs

@@ -72,9 +72,9 @@ pub async fn write_default_execpolicy<P>(policy: &str, codex_home: P) -> anyhow:
 where
     P: AsRef<Path>,
 {
-    let policy_dir = codex_home.as_ref().join("policy");
+    let policy_dir = codex_home.as_ref().join("rules");
     tokio::fs::create_dir_all(&policy_dir).await?;
-    tokio::fs::write(policy_dir.join("default.codexpolicy"), policy).await?;
+    tokio::fs::write(policy_dir.join("default.rules"), policy).await?;
     Ok(())
 }