Local cross-repo packages: agent pointer, cleanup hooks and push guard #31

Make local cross-repo packaging a supported pi workflow that agents reach for when it pays off, and that cleans up after itself (from woksin via SPEC, 2026-10-23). The packaging mechanics stay in the Cratis AI skill cratis-engineering-local-cross-repo-verification (cratis-local-packages.sh); its registry, list and retention gaps are in Cratis/AI#885. pi-setup integrates it:

• Briefs and agents: templates/brief.md and the implementer agents get one line: "when a fix or feature must be proven in a consumer repository, use local packages (the Cratis skill in Cratis repos) instead of asking for a release". The decision guidance stays in the skill (one place, per Single source of truth for AI rules across agents, AGENTS.md, extensions, skills, Cratis AI corpus and other harnesses #30).
• Cleanup: pi-cleanup (daily) and ai-work-lifecycle finish run the skill's expired-stamp cleanup once Local cross-repo verification: pack registry, list, retention and private caches Cratis/AI#885 lands, first as a dry run in the log and then applied under the same safety rules. They never remove a stamp that a live worktree references.
• Push guard: a push or PR in a Cratis repository whose tracked or staged files reference the local feed or a -local./-train. version is blocked. Reuse the skill's check, the same way the trains guard is wired (extension plus opt-in pre-push hook).
• Trains: pi-train records the consumed stamps in the manifest (Local release trains: fold tooling that closes superseded PRs and cancels their runs, tied to local cross-repo packages #25 slice c, Local cross-repo verification: train packages and upstream-first ordering for local release trains Cratis/AI#881).