feat: add justification arg to prefix_rule() in *.rules (#8751)

Adds an optional `justification` parameter to the `prefix_rule()`
execpolicy DSL so policy authors can attach human-readable rationale to
a rule. That justification is propagated through parsing/matching and
can be surfaced to the model (or approval UI) when a command is blocked
or requires approval.

When a command is rejected (or gated behind approval) due to policy, a
generic message makes it hard for the model/user to understand what went
wrong and what to do instead. Allowing policy authors to supply a short
justification improves debuggability and helps guide the model toward
compliant alternatives.

Example:

```python
prefix_rule(
    pattern = ["git", "push"],
    decision = "forbidden",
    justification = "pushing is blocked in this repo",
)
```

If Codex tried to run `git push origin main`, now the failure would
include:

```
`git push origin main` rejected: pushing is blocked in this repo
```

whereas previously, all it was told was:

```
execpolicy forbids this command
```

codex-rs/core/tests/suite/exec_policy.rs

@@ -97,7 +97,7 @@ async fn execpolicy_blocks_shell_invocation() -> Result<()> {
 
     assert!(
         end.aggregated_output
-            .contains("execpolicy forbids this command"),
+            .contains("policy forbids commands starting with `echo`"),
         "unexpected output: {}",
         end.aggregated_output
     );