context: Add Claude background shell mutation-scope plan

Define the observed-lifecycle probe, implementation sequence, acceptance criteria, and context synchronization needed to safely attribute mutations from Claude-managed background Bash commands. The plan covers retaining scopes through terminal completion, recovery and health behavior, hook registration, and Git/trace attribution regressions across T01–T09.

Plan: claude-background-shell-mutation-scope (T01–T09)

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

Author: davidabram