chore: ARC process reduction

Replace the six stage-specific PR templates with:

- `arc-draft.md` for a new ARC;
- `arc-status-change.md` for any status change, with the remaining human
  checks grouped by target status;
- `arc-update.md` for adoption, implementation, and editorial updates.

Machine-checkable gates already run in CI, so the templates only ask for
what a person has to confirm.

The `kind:*` label now comes from the changed files instead of PR template
headings: a new ARC file is `kind:draft`, a status change to `Review`,
`Last Call`, or `Final` gets the matching label, other adoption changes are
`kind:adoption`, and other ARC edits are `kind:editorial`. Labels stay
correct when the author picks the wrong template or edits its heading.

For a `Last Call` to `Final` status change, the transition step reads
`last-call-deadline` from the base branch (Final ARCs must drop the field)
and fails when it is missing or still in the future (UTC).

ARC-0000 now passes every arckit rule without exemptions, so its entry is
removed from `.arckit.jsonc`:

- add the missing `Motivation` section (R:008);
- wrap illustrative `ARC-42` examples in code spans and refer to "this ARC"
  instead of `ARC-0000` (R:036);
- record the 2026-11-02 update.

ARC-0000, the ARC template, and the adoption README now treat the adoption
summary as optional in `Draft` and `Review` and required from `Last Call`,
matching what arckit already enforced. They also describe adoption readiness
as derived from tracked adopters rather than declared, and drop
`last-reviewed`.