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

docs/reference/exit-codes.md

@@ -12,10 +12,6 @@ What each exit code means, and how a caller reacts to it, is [AGENTS.md § Tool
 
 - [`export guidelines`](commands/guideline-export.md#export-guidelines)
 
-### human-intervention-required (`new project` only)
-
-- [`new`](commands/pages.md#new)
-
 ### mass-update
 
 - [`publish`](commands/git.md#publish)