feat: add local date/timezone to turn environment context

[celia-oai]

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>"}]}}

[pakrym-oai]

is this a huge dep?

[celia-oai]

codex says:

• No. In this workspace it is not a huge dependency.

iana-time-zone = "0.1.64" is a small platform abstraction crate for getting the local IANA time zone. Also, that manifest entry is a semver range, so the lockfile currently resolves it to 0.1.65, not exactly 0.1.64.

What matters here:

• On this macOS workspace, cargo tree -p iana-time-zone shows it resolves to just core-foundation-sys.
• The lockfile lists Android/wasm/Windows helpers too, but those are target-specific, not all compiled on this platform.
• It is already in the graph via chrono, so in this repo the incremental cost is effectively negligible.