Add configurable timezone for session timestamps in Codex TUI

[Nicecsh]

Problem

The Codex TUI session picker displays session Created and Updated timestamps in UTC, even when the operating system is configured for China Standard Time (UTC+8 / Asia/Shanghai).

For users in UTC+8, the displayed times are eight hours behind local time and can be confusing when reviewing recent sessions. The relative labels such as “4 minutes ago” are also based on the UTC display context.

Requested feature

Add a persistent timezone setting for session timestamps. For example:

[tui]
timezone = "Asia/Shanghai"

The exact configuration shape can be chosen by the maintainers. The setting should apply to the session picker and other TUI surfaces that display local timestamps.

Expected behavior

• Allow an IANA timezone such as Asia/Shanghai.
• Keep the current UTC behavior as the default for backward compatibility, or provide an explicit local-time option.
• Validate invalid timezone values with a clear configuration error.
• Document the setting and its default.
• Add tests covering UTC and UTC+8 display.

Environment

• OS timezone: Asia/Shanghai (UTC+8)
• Codex interface: TUI session picker
• Observed fields: Created and Updated

This is a usability issue for users working outside UTC and would also benefit users in other time zones.