feat: migrate ManagedAgentGraph to new Runner protocol

ManagedAgentGraph now expects AgentGraphRunnerResult (with GraphMetrics) from
all runners and drives graph-level and per-node LD tracking from result.metrics.
AgentGraphRunner protocol updated to return AgentGraphRunnerResult.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jsonbailey