feat: execpolicy v2 (#6467)

## Summary
- Introduces the `codex-execpolicy2` crate.
- This PR covers only the prefix-rule subset of the planned execpolicy
v2 language; a richer language will follow.

## Policy
- Policy language centers on `prefix_rule(pattern=[...], decision?,
match?, not_match?)`, where `pattern` is an ordered list of tokens; any
element may be a list to denote alternatives. `decision` defaults to
`allow`; valid values are `allow`, `prompt`, and `forbidden`. `match` /
`not_match` hold example commands that are tokenized and validated at
load time (think of these as unit tests).

## Policy shapes
- Prefix rules use Starlark syntax:
```starlark
prefix_rule(
    pattern = ["cmd", ["alt1", "alt2"]], # ordered tokens; list entries denote alternatives
    decision = "prompt",                # allow | prompt | forbidden; defaults to allow
    match = [["cmd", "alt1"]],          # examples that must match this rule (enforced at compile time)
    not_match = [["cmd", "oops"]],      # examples that must not match this rule (enforced at compile time)
)
```

## Response shapes
- Match:

```json
{
  "match": {
    "decision": "allow|prompt|forbidden",
    "matchedRules": [
      {
        "prefixRuleMatch": {
          "matchedPrefix": ["<token>", "..."],
          "decision": "allow|prompt|forbidden"
        }
      }
    ]
  }
}
```

- No match:

```json
"noMatch"
```

- `matchedRules` lists every rule whose prefix matched the command;
`matchedPrefix` is the exact prefix that matched.
- The effective `decision` is the strictest severity across all matches
(`forbidden` > `prompt` > `allow`).

---------

Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/Cargo.lock

@@ -1202,6 +1202,21 @@ dependencies = [
  "tempfile",
 ]
 
+[[package]]
+name = "codex-execpolicy2"
+version = "0.0.0"
+dependencies = [
+ "anyhow",
+ "clap",
+ "multimap",
+ "pretty_assertions",
+ "serde",
+ "serde_json",
+ "shlex",
+ "starlark",
+ "thiserror 2.0.17",
+]
+
 [[package]]
 name = "codex-feedback"
 version = "0.0.0"