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

instructions/claude-code-model-selection.md

@@ -74,8 +74,12 @@ to *make*, not a setting to apply.
    switch at the moment the phase changes - once, without arguing about it, and never as a reason
    to stop work that is already underway.
 <!-- dist:strip-start -->
-   In this repo those moments are named: the `stack-dev` skill breaks for them at its steps 3
-   (design settled, work turns mechanical) and 6 (publish done, the unchecked tail begins).
+   In this repo those moments are named, one per skill rather than both in one: `stack-dev`'s own
+   step 3 breaks for the first (design settled, work turns mechanical), and `stack-dev` itself
+   ends at the publish rather than asking the same session to break out of its own momentum a
+   second time. The second switch lives at the opening of `stack-close`, the skill `stack-dev`
+   hands off to once the publish succeeds (the unchecked tail begins) - a session has to invoke
+   it to reach that step at all, which is the point: nothing left to skip past mid-flow.
 <!-- dist:strip-end -->
 
 2. **Pick a spawned subagent's model by what it does**, via the `Agent` tool's `model:`