Feature request: show the current session title in the TUI footer (proposed /titleon and /titleoff)

Problem

Two related pain points when working with multiple sessions in the terminal:

1. Sessions forked from the same conversation are hard to tell apart. When I fork a conversation to explore different features or tasks, and later switch between those sessions in the same terminal (/sessions, kimi --session), the entries look almost identical — same working directory, similar first prompts. It is easy to resume the wrong session.

2. The current session's title is invisible while you work. Even after renaming a session with /title, the TUI status bar (which shows the working directory and git branch) never displays the title. Inside a session there is no persistent on-screen indicator telling me which conversation I am actually in.

Proposal

Add a session-only display toggle for the footer status bar:

• /titleon — show the current session title in the footer, right after the working directory and git branch badge, e.g. ~/work/kimi-code main fix-auth-flow
• /titleoff — hide it again (the default)

Behavior details:

• Session-only, not persisted: the toggle resets on session switch / /new / fork / logout, but survives /reload within the same session.
• The title is truncated to 40 display columns (width-aware, so CJK titles are capped correctly) with …, so it cannot crowd out the rest of the status bar; it uses the existing two-space separator and the same dim color as the cwd.
• Renaming with /title updates the footer live.
• No git repo → the title simply follows the cwd segment.

Why two new commands instead of /title on|off?

/title <text> currently sets the title verbatim, so /title on is ambiguous — it already means "set the title to on". A dedicated pair /titleon / /titleoff avoids that ambiguity. If maintainers prefer a different shape (a single toggle command, or a /title subcommand with disambiguated syntax), I am happy to adjust — feedback welcome.

I have a complete implementation ready (with unit tests, lifecycle tests, changeset, and en/zh docs updates) and will open a PR referencing this issue.

Locate the TUI footer status bar and existing /title command first, then review the proposed unit tests, lifecycle tests, changeset, and en/zh documentation updates. Done means /titleon and /titleoff control the session-only footer display, including width-aware truncation, live renaming, reset behavior, and the documented tests and changes.