feat: wikitool upstream merge/verify - code procedure for taking a stack update (4.5.0-beta.1, #30)

Files changed:
- AGENTS.md
- CHANGES.md
- VERSION
- instructions/gates.md
- instructions/private-instance.md
- tools/CONTRACT.md
- tools/README.md
- tools/chemenu/cli.py
- tools/chemenu/commands/dist_cmd.py
- tools/chemenu/commands/run_budget.py
- tools/chemenu/commands/upstream_cmd.py
- tools/chemenu/ownership.py
- tools/chemenu/tests/test_upstream_cmd.py

AGENTS.md

@@ -238,7 +238,8 @@ Every `tools/wikitool` call has exactly four outcomes:
    produced.
 
 After the single allowed retry - or immediately, for the non-idempotent commands `new`,
-`log append`, and `publish` - stop and report the exact command and error text to the user.
+`log append`, `publish`, and `upstream merge` - stop and report the exact command and error
+text to the user.
 
 Per-command detail (what exit 1 means, whether the command is atomic, whether a retry is
 safe) is in [tools/CONTRACT.md](tools/CONTRACT.md). A gate refusal is not a validation error -