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.

INSTALL.md

@@ -47,7 +47,7 @@ $EDITOR <project-root>/CLAUDE.md   # paste from templates/CLAUDE.md.fragment
 ```
 
 Everything the plugin does *not* read from project facts is a fixed
-convention (spec dir `docs/specs`, 4-digit naming, the pipeline graph,
+convention (spec dir `docs/specs`, slug-only naming, the pipeline graph,
 …) documented once in `docs/conventions.md` and `docs/pipeline.md` —
 not configurable per project.