AGENTS.md § Gates is the one gate list; cli_contract.GATES and docs verify hold it

The gate list contradicted itself across AGENTS.md, instructions/gates.md and the
command records' gate names. The Rebase-Review Gate is now listed, `new project`'s
exit 42 is no longer a pseudo-gate, gates.md and the explanation page link to the one
list instead of counting, and a register plus a docs verify check keep the three in step.

Files changed:
- AGENTS.md
- CHANGES.md
- VERSION
- docs/explanation/why-gates-are-code.md
- docs/reference/commands/pages.md
- docs/reference/commands/types-instructions-and-docs.md
- docs/reference/exit-codes.md
- instructions/gates.md
- tools/CONTRACT.md
- tools/chemenu/cli_contract.py
- tools/chemenu/commands/docs_verify.py
- tools/chemenu/commands/doctor.py
- tools/chemenu/commands/git_publish.py
- tools/chemenu/commands/new_page.py
- tools/chemenu/errors.py
- tools/chemenu/mcp/server.py
- tools/chemenu/tests/test_cli_contract.py
- tools/chemenu/tests/test_docs_verify.py
- tools/chemenu/tests/test_git_publish.py
- tools/chemenu/tests/test_reference_doc.py

Co-Authored-By: Claude Code <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SnAJ7Z3CpVD3PRbN73QtU2

tools/chemenu/tests/test_reference_doc.py

@@ -268,7 +268,7 @@ def _page_type(description: str = "A fixture type.") -> reference_doc.PageType:
 def tree(tmp_path, monkeypatch):
     real_root = config.ROOT
     monkeypatch.setattr(config, "ROOT", tmp_path)
-    records = {"frobnicate": _record("frobnicate"), "gate keep": _record("gate keep", ("keep",))}
+    records = {"frobnicate": _record("frobnicate"), "gate keep": _record("gate keep", ("mass-update",))}
     monkeypatch.setattr(cli_contract, "all_records", lambda: dict(records))
     monkeypatch.setattr(cli_contract, "GROUPS", GROUPS)
     monkeypatch.setattr(cli, "options_texts", lambda paths: {p: "  --flag  A flag." for p in paths})
@@ -416,7 +416,7 @@ def test_exit_codes_lists_each_gate_and_its_exit_42_causes(tree):
     root, _ = tree
     runner.invoke(app, ["docs", "reference", "--apply"])
     page = (root / "docs/reference/exit-codes.md").read_text(encoding="utf-8")
-    assert "### keep\n\n- [`gate keep`](commands/other.md#gate-keep)" in page
+    assert "### mass-update\n\n- [`gate keep`](commands/other.md#gate-keep)" in page
     assert "- [`gate keep`](commands/other.md#gate-keep): Needs a person" in page
     assert "frobnicate" not in page