Eliminate slice branches — all work commits sequentially on milestone/<MID>
within auto-mode worktrees. No branch creation, switching, or merging
within a worktree. Planning artifacts (.gsd/milestones/) tracked in git
properly instead of being blanket-gitignored then force-added.
Removes ~2,600 lines: ensureSliceBranch, switchToMain, mergeSliceToMain,
mergeSliceToMilestone, shouldUseWorktreeIsolation, getMergeToMainMode,
withMergeHeal, recoverCheckout, fix-merge dispatch/labels, and associated
tests. Adds legacy_slice_branches doctor check, deprecation warnings for
git.isolation and git.merge_to_main preferences.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
