[FEATURE] Show background shell (and subagent) output in the /tasks detail panel

[edenreich]

Summary

In /tasks, selecting a background shell (or subagent) row shows only metadata - the command (Detail:), status, and elapsed time - but not the shell's output. There is no way to see, let alone stream, the captured stdout/stderr of a background shell from /tasks. A2A tasks already render their result (renderFinalResult, internal/ui/components/task_management_view.go:808), so shells/subagents feel inconsistent.

The data is not currently plumbed to the view: the /tasks view consumes the supervisor Snapshot() as domain.TrackedJob, which carries Meta / Status / CompletedAt / LastNote (internal/domain/background_job.go:133-137) - metadata and a short last-note, but not the captured output stream. A background shell's full output currently flows into the conversation when the job finishes (via the supervisor completion note), not into the /tasks detail panel.

Requested: when a background shell/subagent row is selected in /tasks, show its output in the detail panel - ideally streamed live for a running shell, and the captured output (or a tail) for a finished one - bringing shells/subagents to parity with the A2A "Final Result" panel.

Acceptance Criteria

• Selecting a running background-shell row in /tasks shows its output, updated live as it streams.
• Selecting a completed background-shell row shows its captured output (or a bounded tail with a way to see more).
• The same applies to subagent rows (show the subagent's output/result).
• Output rendering is bounded (truncation/scroll) so a chatty shell does not blow up the panel.
• Plumb the output to the view: extend the supervisor snapshot / TrackedJob (or add a lookup by job id) so the detail panel can read a job's output without routing through the conversation.

Reported on macOS, infer chat TUI, against the new per-kind /tasks tables.