Feat: Group subagent sessions under their parent in abctl

Background

PR #1062 added X-Session-Id to the default session.id_headers, enabling OpenCode sessions to appear in abctl. OpenCode assigns a distinct session ID to each subagent, which is good for granular tracking, but results in many top-level entries in the session list when a multi-agent workflow runs.

For agents that support a parent session identifier, like X-Parent-Session-Id in OpenCode, capturing this identifier would allow abctl to reflect the agent hierarchy.

Feature request

Use the parent session identifier to build a session hierarchy in abctl:

• The top-level view shows only root (parentless) sessions
• A root session can be drilled into to reveal its subagents, recursively

This would require changes at three layers: the proxy (capture the parent header), the session API (expose the parent relationship), and the abctl TUI (collapsible tree in the session list).

Start by locating the proxy, session API, and abctl TUI session-list entry points named in the request, and trace how X-Session-Id is currently captured and exposed. Done means parent identifiers are available to the session view, root sessions are shown initially, and subagents can be expanded recursively in the hierarchy.