Add revert!/discard and rebuild nodes#edit around the new hierarchy

revert! discards exactly the topmost non-empty layer -- autosave if
present, else draft -- and reveals whatever's beneath it, releasing the
lock only once nothing is left to protect. Guards against destroying a
brand-new, never-published node's only draft, which would violate the
(head | draft) invariant every other method here assumes holds; the
view's Destroy/Discard button is gated the same way.

nodes#edit now calls lock_for_editing! instead of find_or_create_draft,
and always displays autosave || draft || head, resurrecting an
abandoned session's unsaved content by default with an explicit flash
explaining what's shown and how to get back to the last saved version.

The view drops content_for :subnavigation entirely: Show becomes
"Unlock + Back", Preview stays a plain link, metadata's own <details>
already replaced the old toggle, and Publish moves off this page for
good, per the earlier decision to manage the publish lifecycle
entirely from nodes#show. Save Draft and Save + Unlock + Exit appear
both above and below the form, given the body field alone runs 600px
on desktop.

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