#807 Stage 8 epic: trace UI + docs (#agentic)

Dashboard Agent-trace tab rendering GET /workflows/<id>/trace (agent tree, per-node tokens/cost/duration, spans) under highway-dashboard. docs/AGENTS.md: run_goal lifecycle, catalog, delegation/call_agent, sessions, memory, HITL (edit-args + timeout), specs, fork/time-travel, progress stream, harness. Brief per CLAUDE.md. Ungated (final stage).