docs: never commit prerelease versions; derive from registry, bump files, and tags

Major revision of the channels design around one principle: git carries
inputs (bump files) and stable outputs only. Prerelease versions exist
solely in the registry and git tags.

- package.json stays at the last stable version on channel branches
- release PR becomes a pure file-move PR; computed versions go in the
  PR title and merge commit message (advisory; registry wins at publish)
- counters derived from registry floor (max published + 1) — immune to
  abandoned cycles, force-resets, and re-runs
- every publish recomputes the full cycle; lockstep republish + exact
  pins make the @next set coherent by construction
- promotion is now just a merge; no suffix stripping, no rc versions
  ever on main, no version conflicts on main <-> channel syncs
- generalized pending rule enables channel graduation (alpha -> beta)
- no CHANGELOG.md on channels; GitHub releases + render-on-demand via
  bumpy status; consolidated stable entry written once at promotion
- publish mechanics specified: trigger via channel-dir diff since last
  tag, tag-on-SHA idempotency, gitHead-based partial-failure resume
- document build-time version baking caveat (rewrite before build)
- reserve schema room for future stable/maintenance channels

Author: theoephraim