follow-up(agent-debugger): richer trace schema (node IDs, lineage, token/timing, replay checkpoints)

[jaylfc]

Follow-up to #485 (agent debugger, closes #131). The v1 debugger ships with a minimal trace event schema. A CodeRabbit review flagged that the full debugger contract would benefit from a richer schema:

• stable node IDs + parent/child lineage for each trace event
• token usage + timing per step
• replay checkpoints (so step-back / re-run from a point is possible)

Deferred from #485 to keep that PR's scope contained. Pick up when extending the debugger beyond basic SSE step-through.