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/core/tests/suite/approvals.rs

@@ -1633,7 +1633,7 @@ async fn approving_execpolicy_amendment_persists_policy_and_skips_future_prompts
         .await?;
     wait_for_completion(&test).await;
 
-    let policy_path = test.home.path().join("policy").join("default.codexpolicy");
+    let policy_path = test.home.path().join("rules").join("default.rules");
     let policy_contents = fs::read_to_string(&policy_path)?;
     assert!(
         policy_contents