refactor(pipeline): specs/plans are gitignored working files, never committed

Per-cycle specs and plans stop being git-tracked commit-then-git-rm
artefacts and become git-ignored working files: created on disk for the
active cycle, read across sessions from the working tree, and shell-rm'd
at cycle close by audit. Nothing is committed, so there is no
git-history copy — the durable record of a cycle's intent is the design
ledger only, and no durable artefact (code, test, doc, ledger) may cite
a spec/plan by number or path.

- conventions/pipeline/design/README: invert the git-tracked -> git-rm
  -> git-history-recovery model to gitignored -> shell-rm -> ledger-only.
- naming: drop the NNNN file counter; files are slug.md (fieldtest specs
  keep a fieldtest- prefix as the cycle-close discriminator).
- cycle number: separated from the (now-gone) file number, sourced from
  the feat(NNNN)/audit(NNNN) commit subjects, which cycle commits carry.
- specify/planner/fieldtest/fieldtester: no commit step; fieldtest splits
  fixtures (committed as code) from its spec (gitignored), with the
  cross-cycle rule that a fieldtest- spec belongs to the next cycle.
- boss: auto-sign auditability moves from the (boss-signed) commit
  subject to the run's reference issue; rollback reverts only code.
- audit: new Step 5 shell-rm's the cycle spec+plan after the architect
  reads them, sparing fieldtest- specs.

Companion machine config (outside this repo): the global git excludes
(~/.config/git/ignore) gains docs/specs/ and docs/plans/ so the ignore
applies to every project at once.

docs/design.md

@@ -85,16 +85,18 @@ The plugin owns:
 The constants that used to be configurable but were the same in
 every project are now plugin conventions, named directly in skill
 and agent bodies and documented once in `conventions.md`:
-spec dir `docs/specs`, plan dir `docs/plans`, 4-digit per-directory
+spec dir `docs/specs`, plan dir `docs/plans`, slug-only `slug.md`
 naming, the vocabulary cycle / iteration / milestone / contract,
 standing reading (`CLAUDE.md` + `git log -10`), git discipline
 (only-orchestrator commits, main sacrosanct), Gitea + `closes #N`,
 and the whole pipeline graph (see `pipeline.md`).
 
-Specs and plans are ephemeral active-cycle artefacts — the two
-directories stay git-tracked and hold only what is in flight; at cycle
-close each is `git rm`'d from the repo, kept only in git history, never
-as a durable file (see `conventions.md` § Lifecycle).
+Specs and plans are ephemeral active-cycle working files — the two
+directories are git-ignored and hold only what is in flight; at cycle
+close each is shell-`rm`'d (never committed, so nothing to `git rm` and
+no git-history copy). The durable record of a past cycle is the design
+ledger only, and no durable artefact cites a spec/plan (see
+`conventions.md` § Lifecycle).
 
 ## Project facts (project-specific)