Remove the separate CheckpointSaved event — CheckpointCompleted now fires in
both git and non-git paths. git_commit_sha is Optional (None when git is
disabled or for start nodes). The CheckpointSaved hook event is preserved
unchanged for backward compat with user hook configs.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Rename GitCheckpoint/GitCheckpointFailed to CheckpointCompleted/CheckpointFailed
to separate checkpoint lifecycle from git operations. Add 7 new granular git
events: GitCommit, GitPush, GitBranch, GitWorktreeAdd, GitWorktreeRemove,
GitFetch, GitReset. Emit at all relevant call sites in engine.rs and parallel.rs.
Update push helpers to return bool for GitPush success tracking.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>