bug(claude): verify Claude subagent tracking against current Claude Code (companion to #340) #1470

Companion to #340 (Codex subagents missing from state), requested by the owner on 2026-09-27.

Claude subagents are tracked by src/main/subagents/SubAgentWatcher.ts, which polls <sessionDir>/subagents/, together with subagentState.ts (a derive-and-drop accumulator). The feed rows come from src/providers/claude/renderer/adapters/collaboration.ts, which recognizes only the built-in Agent grammar captured in recordings made during PR #555.

What to verify on current Claude Code (2.1.283):

1. Subagents still write under <sessionDir>/subagents/ in the layout the watcher expects, including background and parallel Agent runs.
2. The Agent/Task tool input and result grammar still matches collaboration.ts (subagent_type, the result shape, and any new fields).
3. Parent→child linkage, status and tool counts appear for every current subagent form.

Method: record a fresh Claude session that spawns a foreground, a background and a parallel subagent. Compare the transcripts and subagents/ files with what the watcher and adapter parse, and write fail-first tests from that recording for any gap.

Refs #340.