stack-dev/stack-close skill split; publish stack-machinery note; model-selection fix (#47 Block 2)

Files changed:
- CHANGES.md
- VERSION
- instructions/claude-code-model-selection.md
- instructions/dev/stack-close/SKILL.md
- instructions/dev/stack-dev/SKILL.md
- tools/CONTRACT.md
- tools/chemenu/commands/git_publish.py
- tools/chemenu/tests/test_git_publish.py

tools/chemenu/commands/git_publish.py

@@ -423,6 +423,41 @@ def counted_files_of(
     return [change for change in changes if not is_exempt(change.path, prefixes)]
 
 
+# The same scope `version-parts.md` names for the stack version itself -
+# "tools/, types/, instructions/, AGENTS.md and the contracts" - reused here
+# to decide whether a publish's changeset falls under it. Not a copy of that
+# rule: version-parts.md states the scope in prose for a human choosing a
+# version part, this instantiates the same boundary in code for a different
+# question (does this publish deserve the closing-phase reminder below).
+STACK_MACHINERY_PREFIXES = ("tools/", "types/", "instructions/")
+STACK_MACHINERY_NAMES = ("AGENTS.md",)
+
+STACK_MACHINERY_NOTE = (
+    "Note: this publish touched stack machinery. What a stack-dev session "
+    "does next - closing prose, a changelog entry's accuracy, whether a "
+    "docs/ page went stale - is not covered by docs verify, instructions "
+    "verify, or pytest. No tool checks it; a session has to."
+)
+
+
+def touches_stack_machinery(changed_files: list[str]) -> bool:
+    """Whether `changed_files` includes a path under `version-parts.md`'s
+    scope for the stack version - `tools/`, `types/`, `instructions/`,
+    `AGENTS.md`, or any `<stage>/CONTRACT.md`. A publish in this class is,
+    by construction of the `stack-dev`/`stack-close` split, always followed
+    by the unchecked closing phase - `STACK_MACHINERY_NOTE` times a reminder
+    to land exactly there, for any session, not only one that read the
+    skill that names it."""
+    for path in changed_files:
+        if path in STACK_MACHINERY_NAMES:
+            return True
+        if path.endswith("CONTRACT.md"):
+            return True
+        if path.startswith(STACK_MACHINERY_PREFIXES):
+            return True
+    return False
+
+
 YES_REMOVED_MESSAGE = (
     "--yes no longer exists. The Mass-Update Gate is cleared with `--confirm <token>`, and the "
     "token comes from the gate's own refusal output - run this command without it first to see "
@@ -1138,3 +1173,5 @@ def publish_command(
         },
     )
     success(f"Published changes to {remote}/{branch}." if push else "Committed changes (not pushed).")
+    if touches_stack_machinery(changed_files):
+        typer.echo(STACK_MACHINERY_NOTE)