feat: add local date/timezone to turn environment context (#12947)

## Summary

This PR includes the session's local date and timezone in the
model-visible environment context and persists that data in
`TurnContextItem`.

  ## What changed
- captures the current local date and IANA timezone when building a turn
context, with a UTC fallback if the timezone lookup fails
- includes current_date and timezone in the serialized
<environment_context> payload
- stores those fields on TurnContextItem so they survive rollout/history
handling, subagent review threads, and resume flows
- treats date/timezone changes as environment updates, so prompt caching
and context refresh logic do not silently reuse stale time context
- updates tests to validate the new environment fields without depending
on a single hardcoded environment-context string

## test

built a local build and saw it in the rollout file:
```
{"timestamp":"2026-02-26T21:39:50.737Z","type":"response_item","payload":{"type":"message","role":"user","content":[{"type":"input_text","text":"<environment_context>\n  <shell>zsh</shell>\n  <current_date>2026-02-26</current_date>\n  <timezone>America/Los_Angeles</timezone>\n</environment_context>"}]}}
```

codex-rs/protocol/src/protocol.rs

@@ -2133,6 +2133,10 @@ pub struct TurnContextItem {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub turn_id: Option<String>,
     pub cwd: PathBuf,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub current_date: Option<String>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub timezone: Option<String>,
     pub approval_policy: AskForApproval,
     pub sandbox_policy: SandboxPolicy,
     #[serde(skip_serializing_if = "Option::is_none")]
@@ -3361,6 +3365,8 @@ mod tests {
         let item = TurnContextItem {
             turn_id: None,
             cwd: PathBuf::from("/tmp"),
+            current_date: None,
+            timezone: None,
             approval_policy: AskForApproval::Never,
             sandbox_policy: SandboxPolicy::DangerFullAccess,
             network: Some(TurnContextNetworkItem {