fix(pipeline): complete the gitignored-specs refactor (review gaps)

An adversarial review of 08bc2aa found three gaps:

- templates/CLAUDE.md.fragment still described the OLD git-tracked /
  git-rm-at-close model verbatim — the worst place to miss it, since new
  projects import this fragment into their CLAUDE.md. Rewritten to the
  gitignored / shell-rm / ledger-only model.
- INSTALL.md still cited "4-digit naming" — now slug-only.
- The fieldtest cross-cycle deletion rule was self-defeating: it keyed
  the spare/delete decision on the permanent fieldtest- prefix, which
  cannot encode "spare once, then delete." Because fieldtest runs only
  AFTER audit closes, the fresh fieldtest spec never exists at the
  cycle-close sweep, so the rule is simply "audit discards every
  working-tree spec/plan"; the spent fieldtest spec present at N+1's
  close was already consumed and goes with the rest. The prefix is now
  only a working-tree label, not a delete discriminator. Reconciled
  across audit, conventions, pipeline, fieldtest, fieldtester.

docs/conventions.md

@@ -77,9 +77,10 @@ and cited by nothing (see § Lifecycle), they hold at most the active
 cycle's handful of files, so a slug is already unique enough — the old
 scan-for-the-next-free-number ritual bought nothing and is gone. A
 `fieldtest`-produced spec takes a `fieldtest-` prefix
-(`fieldtest-slug.md`): that prefix is the discriminator the cycle-close
-sweep uses to spare it, because it belongs to the next cycle (see
-`../fieldtest/SKILL.md`).
+(`fieldtest-slug.md`) so it is recognisable in the working tree as the
+next cycle's planning input rather than this cycle's own spec — it is
+written after audit's cycle-close sweep and discarded by the next
+cycle's sweep (see `../fieldtest/SKILL.md`).
 
 **The file name is not the cycle number.** The **cycle number** that
 appears in commit subjects (`feat(NNNN)`, `audit(NNNN)`) and in `git