Capture Claude child transcripts before terminal hooks

When Claude Code capture hooks supply only the main transcript path, inspect the current session's native subagent folder instead of waiting for SubagentStop. Validate regular files under the allowed root and check declared session/agent identity, then capture appended bytes under each child's original stream identity and parent link. This makes available usage observable during long child runs without reimporting unchanged bytes or mixing unrelated sessions. Keep discovery scoped to the current family and retain partial coverage: hooks, debouncing, and missing host installations still bound what can be observed.

Applies when: A Claude Code collector relies on hook-provided paths to capture background subagents. Keywords: Claude Code telemetry subagent background hooks transcript discovery usage