Show active and dead records in the subagents command

[nisan1101]

What to build

Expand /subagents into a newest-first branch-history picker. Active subagents can still open their live viewer, while completed, failed, and killed records remain visible as disabled rows that cannot be selected.

Acceptance criteria

• /subagents displays every record known on the active branch rather than only active children.
• Rows show the subagent handle and one user-facing status, ordered by creation time descending.
• starting, running, and waiting rows are selectable and retain the existing live-viewer behavior.
• completed, failed, and killed rows are visibly disabled and cannot be selected or open a viewer.
• A branch containing only terminal records still opens the history picker instead of reporting that no active subagents exist.
• Cancelling the picker performs no action.
• Existing automatic viewer selection and manual follow-tail fallback remain unchanged for selectable active records.
• Outside interactive mode, the command continues to report that interactive mode is required and performs no action.
• Behavioral tests verify newest-first ordering, mixed active and terminal rows, dead-only history, disabled-row navigation, active selection, and cancellation.

Blocked by

• List all branch subagents through a parent tool #4