v1.51.0 — a Next Steps procedure at the end of every push

A Next Steps procedure at the end of every push. Sessions kept ending with the
work that only the maintainer could do — pushing a tag, publishing a release,
bumping the fleet — mixed in with loose follow-up ideas, and the 1.45.0 and
1.50.0 tag steps lived only in chat scrollback (ADR-0005). Now every summary,
pull request, and handoff ends with the same ordered checklist that says who
acts on each step, and a checker holds it to that shape.

Added

• Every push ends with a Next Steps procedure (AI_WORKFLOW.md, "Next
  Steps Procedure"; ADR-0005). The closing summary, the pull request
  description, and docs/AGENT_HANDOFF.md now end with the same numbered
  checklist of what happens next, in order. Every step says who acts —
  HUMAN, AGENT, or AUTOMATED — and the steps only a person can do
  (connecting hardware, entering a credential, a UI-only setting, an
  approval) are named up front in a **Human action required:** line and
  then described: why a human, what you need, the procedure, and how you know
  it is done. The procedure is never empty: with nothing pending it still
  offers a suggestion. Trivial changes do not skip it.
• scripts/check_next_steps.sh verifies that shape in
  docs/AGENT_HANDOFF.md, or in a pull request body piped with --file -.
  Warn by default, --strict to block; paired with
  scripts/test_check_next_steps.sh. tests.yml runs it --strict on this
  repository's handoff, and constitution-compliance.yml runs it in warn
  mode on adopters' handoffs, skipping pins that predate it.
• Every agent instruction file, both pull request templates, and the
  handoff template carry the rule. tests.yml enforces the instruction
  files with an explicit check_instruction_templates.sh --anchor; the
  checker's default anchors are unchanged, so no adopter's strict run
  changes behavior in this release.