fix: include nested Claude subagent usage

[raphberube]

When Claude Code executes a Workflow, its subagent transcripts can be stored under subagents/workflows/<workflow-id>/. The collector previously scanned only the immediate subagents directory and required metadata files, so these nested workflow agents could be missing from abtop while actively working. Parent-session token totals also excluded subagent usage, leaving reported usage incomplete during workflow execution.

This change discovers those agents and includes their input, output, cache-read, and cache-write tokens in parent-session totals. Usage continues updating while subagents work, even when the parent transcript is unchanged.

Changes:

• Recursively discover agent-*.jsonl transcripts, using the filename when metadata is unavailable.
• Parse appended data incrementally and cache usage per transcript.
• Handle partial lines, truncation, and file replacement; retain collected totals when completed-agent files disappear.
• Skip symlinks and workflow journals, and sort agents by transcript path for stable display ordering.
• Preserve the parent’s context percentage and turn count.

Regression coverage includes nested discovery, updates without parent activity, repeated polling without double-counting, delayed metadata, transcript lifecycle handling, stable ordering, and cache cleanup.