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.

fieldtest/agents/fieldtester.md

@@ -211,14 +211,15 @@ not merged.
 
 ### Phase 5 — Write the spec, hand back
 
-Write the fieldtest spec under `docs/specs` with a name
-following the `NNNN-slug.md` convention (4-digit prefix,
-per-directory; see `docs/conventions.md`). Use the spec
-template below. Leave all artefacts (the fixtures and the
-spec file) in the working tree as unstaged changes. You do
-NOT commit — the orchestrator commits after reading the
-end-report (suggested commit subject:
-`fieldtest: <cycle> — <N> examples, <K> findings`).
+Write the fieldtest spec under `docs/specs` named
+`fieldtest-slug.md` (the `fieldtest-` prefix is mandatory — it is the
+discriminator that spares the spec from the cycle-close sweep; no numeric
+prefix, see `docs/conventions.md` § Naming). Use the spec
+template below. Leave all artefacts in the working tree as unstaged
+changes. You do NOT commit — the orchestrator commits the **fixtures**
+as code after reading the end-report (suggested subject
+`fieldtest: <cycle> — <N> examples, <K> findings`); the fieldtest
+**spec** is git-ignored and is never committed.
 
 Report `DONE` with the carrier (see Output format).