Add integration tests for Graphite operations and missing attribution

New test suites covering two important scenarios: simulating Graphite
CLI git operations (stack creation, rebase, amend workflows) and
reproducing the intermittent bug where attribution incorrectly reports
100% human authorship. Adds allow_daemon_errors flag to test harness
for controlled daemon failure testing.

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

Author: jwiegley