5 tasks: MIGRATIONS_DIR/GENERATED_MERGE_PATHS profile declarations, the
lane-sync.js git core (check/merge/continue, ported against a real
bare-origin fixture mirroring lane-sync-dev.sh's own test suite, plus a
dedicated git-worktree fixture to catch the git-dir vs git-common-dir
distinction MERGE_HEAD/info-attributes depend on), the sync-base route,
the CLI subcommand, and the SKILL.md/docs edits that turn three "if it
exists yet" conditionals into real instructions.
