Lift global branch coverage past the 80% gate (was 79.53%). Adds error
and fallback path tests: missing-session/unknown-id throws, findRolloutById/
findSessionInfoById, direct file targets, objective truncation, model
fallbacks, corrupt-line skip, mtime activity fallback, and the real
resolveGitBranch path outside a repo.
codex-worktree.js branch 52.8%->78.3%; global branch 80.04%.