List all branch subagents through a parent tool

[nisan1101]

What to build

Give the parent agent a list_subagents tool that reports every subagent known on the active branch, including long-dead checkpointed records, through one stable newest-first view.

Acceptance criteria

• list_subagents is available to the parent and accepts no identifier or polling parameters.
• The tool returns every record on the active branch, including active and checkpointed terminal records.
• Each record includes its full UUID, display name, creation time, and user-facing status.
• User-facing statuses are limited to starting, running, waiting, completed, failed, and killed; the internal finalization phase is never exposed.
• Records are sorted by creation time descending, with the newest subagent first.
• An empty branch produces a clear successful response rather than an error.
• Results remain consistent after Pi restart, session resume, explicit kill, natural completion, failure, and committed tree navigation.
• The tool is excluded from child sessions alongside the other parent-only orchestration tools.
• Behavioral tests verify field completeness, ordering, every status, empty history, branch isolation, and recovered terminal records.

Blocked by

• Checkpoint subagent history and reconcile tree navigation #3