feat(tui): support multi-line output from status_line.command (align with Claude Code statusLine contract)

Summary

status_line.command currently renders only the first stdout line of the user command. Claude Code's statusLine contract (which the seam deliberately mirrors) renders all stdout lines, enabling multi-line HUD-style status bars such as claude-hud. Supporting multi-line output would let users build much richer status displays without forking.

Current behavior

runStatusLineCommand in apps/kimi-code/src/tui/utils/status-line-command.ts truncates stdout at the first \n and discards the rest:

child.stdout?.on('data', (chunk: string) => {
  if (stdout.includes('\n')) return; // first line is complete
  ...
});

and FooterComponent.render() in apps/kimi-code/src/tui/components/chrome/footer.ts hardcodes line 2 to the transient hint + context readout.

Desired behavior

When the configured command emits multiple lines, the footer renders them in order (e.g. up to a small cap like 5 lines), followed by — or replacing — the built-in context line, at the user's option. Single-line commands behave exactly as today, so this is fully backward compatible.

Use case

A HUD-style footer that shows, without crowding one line:

• Line 1: model, cwd:branch, permission/plan mode
• Line 2: context bar + quota/rate-limit windows (fetched from /coding/v1/usages)
• Line 3: session duration, background task counts, external daemon status

Today all of this must be crammed into a single truncated line or dropped.

Suggested implementation (happy to PR)

1. status-line-command.ts: keep the full stdout (still byte-capped) instead of slicing at the first newline.
2. footer.ts render(): when the custom output contains \n, render each line via truncateToWidth, and keep appending the built-in context line unless the custom output already covers it (or gate this behind a config flag such as [status_line].multiline = true).

Thanks for considering!

Start with runStatusLineCommand in apps/kimi-code/src/tui/utils/status-line-command.ts and FooterComponent.render() in apps/kimi-code/src/tui/components/chrome/footer.ts. Trace the current stdout byte cap, newline handling, and truncateToWidth usage before deciding how multiline output interacts with the built-in context line. Done means preserving single-line behavior while rendering capped command output lines in order without exceeding the footer width.