refactor(rpiv-pi): split built-in-workflows' leaf clusters into built-ins/* modules

Shrink the 3.4k-line built-in-workflows monolith to its graph definitions by
relocating the helper clusters verbatim into focused modules behind the
built-ins/ barrel: gates, grade-panel, priors, reconcile, citations,
plan-cite, plan-phases, slices, slice-checks, scope-checks, and goal-baseline
(the goal-capture channel + run-start git baseline).

Update the cross-package comment references the move forces (loop-waves,
worktree-digest) and fix the built-in count comment in
register-built-in-workflows - four workflows including ship.