feat(tui): show running subagents in the prompt status row - #53612

Issue for this PR

Closes #53611

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Lists running subagents on the left of the row under the prompt, so you can see one is still working after its inline task line scrolls away.

• While the parent is busy they follow esc interrupt; when it's idle (background subagents) they take the place of the directory, which comes back once they finish.
• Each shows a spinner and its description: up to 40 characters for one, 24 each for two. Three or more collapse to ⠧ 3 subagents.
• Click one to open its session. Clicking the count opens the oldest.

"Running" comes from the child session's status (busy or retry), not the parent's task tool part. Background tasks complete their part as soon as the child spawns. Also, with at least one provider (cursor-opencode-provider), the running part doesn't carry metadata.sessionId until it completes. Child sessions by parentID are what the existing subagent navigation already uses.

The first commit adds a session_prompt_footer plugin slot at the same spot, so plugins can put things in this row too.

How did you verify your code works?

Unit tests for the running-subagent selection in packages/tui/test/routes/running-subagents.test.ts, the TUI test suite, and bun typecheck in packages/tui and packages/plugin. Ran it locally with two subagents: each showed while it ran and the row cleared when both finished.

Screenshots / recordings

Two subagents run in parallel; both show after esc interrupt while they work, and the row goes back to the directory when they finish. Played at 2x. (mp4)

Running	Finished

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR