feat(tui): statusline — let custom line coexist with built-in tips; expose background task counts in payload

Problem

Two limitations of the [status_line] command integration:

1. All-or-nothing rendering. When a custom statusline command is configured, its output replaces the entire footer line 1 — the built-in rotating tips (the feature-discovery slot) disappear completely. Users who want a custom status line lose the tips slot for good.
2. Payload gaps. The JSON snapshot fed to the external command includes model/cwd/git/permission/context/session, but not the background task counts (bash tasks / subagents) that the built-in slots can show as [N tasks running]. A custom statusline therefore cannot surface "background work is running" — arguably the most glanceable signal.

Proposal

1. When the custom line is present, keep rendering the built-in tips on the right side if space allows (custom content left, tips right).
2. Add backgroundTasks and backgroundAgents to the statusline payload.

Both changes are backward-compatible: existing configs behave the same unless they use the new payload fields.

Context

We run both as small local patches today (custom line + tips coexistence; payload enrichment) and would happily delete them. If the direction is acceptable (/approve), I can open a PR.

Locate the TUI statusline implementation for the [status_line] command integration and the JSON snapshot sent to external commands. Verify that custom content and built-in tips can render together when space allows, and that the payload exposes backgroundTasks and backgroundAgents without changing existing configurations.