feat(tui): show message timestamps and response duration

What feature would you like to see?

Show message timestamps and assistant response duration in the Kimi Code CLI TUI without reducing the usable transcript width, including correct behavior when reopening historical sessions.

Motivation

The current timestamp layout and data flow have several limitations:

1. Indented timestamps leave unused horizontal space under message bullets, which is especially noticeable for code blocks and Markdown tables.
2. Assistant messages do not show how long the complete response took, including streaming and tool execution.
3. Timestamp visibility cannot be reloaded at runtime.
4. Reopened sessions need to distinguish records with persisted timing from legacy records that contain no timing information.

Expected behavior

• Render timestamps on a single header line above the message body:
  • User: ✨ 14:23:45 for today, or ✨ 2026-08-05 14:23:45 for an earlier date.
  • Assistant: ● 14:23:57 (took 3s).
• Keep message content unindented so it can use the full terminal width.
• Measure assistant duration from the persisted response start and completion times.
• Add show_timestamp to ~/.kimi-code/tui.toml, defaulting to true.
• Apply show_timestamp changes through /reload-tui without restarting the CLI.
• When reopening a session:
  • Show timestamps and duration when the journal contains the required timing records.
  • Hide timing when legacy records do not contain it.
  • Never substitute the current wall-clock time for missing historical timing.

Compatibility requirements

• Treat timing as replay/display metadata rather than model-facing conversation content.
• Keep existing session journals readable.
• Do not change the Web transcript or message API behavior as part of this CLI-only feature.

Acceptance criteria

• Live user and assistant messages render the configured timestamp layout.
• Assistant duration updates after the response finishes.
• Historical sessions restore persisted timing accurately.
• Legacy sessions without timing render without timestamps or duration.
• Toggling show_timestamp and running /reload-tui updates the current transcript.
• Unit and replay tests cover live formatting, persisted timing, and legacy missing-time behavior.

Additional information

No response

Start with the TUI message rendering, session journal replay, and configuration flow, using /reload-tui as the runtime entry point. Trace how live and historical records reach the transcript, then use the requested unit and replay tests to verify formatting, persisted timing, legacy records, and runtime visibility changes.