plans: Add Claude model attribution state implementation plan

Production Claude diff traces often lack model identity because PostToolUse metadata and asynchronous transcript lookup are frequently unavailable at hook time. Define a Claude-specific latest-model state register to capture lifecycle observations and preserve direct > transcript > state > NULL attribution precedence.

The plan keeps state local-only, isolates subagents, guards observations by timestamp, and avoids restoring the retired generic session-model abstraction.

Plan: claude-model-attribution-state (T01-T06)

Co-authored-by: SCE <sce@crocoder.dev>

Author: davidabram