fix(tui): only show background work in footer status

[kitlangton]

Problem

The V2 prompt footer shows live subagent and shell counts for foreground work that is currently blocking the session. The count appears while the user is already looking at that work, then repeatedly appears and disappears as foreground tools start and settle.

The footer should communicate work that has been moved out of the main interaction, not duplicate foreground execution state.

Current counting in packages/tui/src/component/prompt/index.tsx includes every running descendant session and every shell record associated with the session, without checking whether the work was explicitly backgrounded.

Expected

• Show footer counts only for subagents and shells explicitly moved to the background.
• Do not show counts for foreground tool calls that the session is currently awaiting.
• Avoid footer flicker as ordinary foreground work starts and finishes.
• Keep the background-work composer tabs/counts aligned to the same definition of background work.

Reproduction

1. Start a foreground subagent or shell tool call in the V2 TUI.
2. Leave it running or waiting for completion.
3. Observe 1 subagent · 1 shell in the prompt footer despite neither task having been backgrounded.

[opencode-agent]

Latest huddle decision: footer/shell indicator should only show work explicitly moved to background, not foreground tool calls the user is already watching. Source discussion: https://slack.com/archives/C0BE69AHCQP/p1783640722114279