Feature Request: expose caller/session timezone inside Codemode sandbox execution

## Problem

Codemode's sandboxed JS execution has no way to know the timezone of the
end-user/session driving the agent. `new Date()` inside the sandbox only
gives server/UTC time, with no IANA timezone context passed through from
`useAgent`/the calling session.

This breaks any date-boundary logic computed inside codemode (e.g. "today",
"this week") for users outside UTC: the sandbox's "today" is a UTC calendar
day (or a UTC-anchored rolling window), which can diverge from what the
end-user considers "today" in their own timezone — off by several hours
depending on time of day and offset.

## Request

Expose the caller's timezone (an IANA string, e.g. from `useAgent` config/
session) to the codemode sandbox — either:
- as a pre-run injected global/context value the sandboxed code can read, or
- as an option on the codemode execution call (e.g. `codemode.run(code, { timezone })`)

so date-boundary math done inside sandboxed code can be computed correctly
relative to the user's local calendar day instead of always UTC.

## Context

We hit this in production: an agent used codemode to compute a "today"
transaction window server-side (UTC-anchored), while our own frontend
computed "today" using the workspace's business timezone. The two disagreed
by a few hours around midnight UTC, producing different transaction counts
for what both surfaces called "today".