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/execpolicy/tests/basic.rs

@@ -54,7 +54,7 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser.parse("test.codexpolicy", policy_src)?;
+    parser.parse("test.rules", policy_src)?;
     let policy = parser.build();
     let cmd = tokens(&["git", "status"]);
     let evaluation = policy.check(&cmd, &allow_all);
@@ -129,8 +129,8 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser.parse("first.codexpolicy", first_policy)?;
-    parser.parse("second.codexpolicy", second_policy)?;
+    parser.parse("first.rules", first_policy)?;
+    parser.parse("second.rules", second_policy)?;
     let policy = parser.build();
 
     let git_rules = rule_snapshots(policy.rules().get_vec("git").context("missing git rules")?);
@@ -194,7 +194,7 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser.parse("test.codexpolicy", policy_src)?;
+    parser.parse("test.rules", policy_src)?;
     let policy = parser.build();
 
     let bash_rules = rule_snapshots(
@@ -259,7 +259,7 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser.parse("test.codexpolicy", policy_src)?;
+    parser.parse("test.rules", policy_src)?;
     let policy = parser.build();
 
     let rules = rule_snapshots(policy.rules().get_vec("npm").context("missing npm rules")?);
@@ -323,7 +323,7 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser.parse("test.codexpolicy", policy_src)?;
+    parser.parse("test.rules", policy_src)?;
     let policy = parser.build();
     let match_eval = policy.check(&tokens(&["git", "status"]), &allow_all);
     assert_eq!(
@@ -367,7 +367,7 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser.parse("test.codexpolicy", policy_src)?;
+    parser.parse("test.rules", policy_src)?;
     let policy = parser.build();
 
     let commit = policy.check(&tokens(&["git", "commit", "-m", "hi"]), &allow_all);
@@ -403,7 +403,7 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser.parse("test.codexpolicy", policy_src)?;
+    parser.parse("test.rules", policy_src)?;
     let policy = parser.build();
 
     let commands = vec![