feat: stack development in three phases - stack-dev (design), stack-build, stack-close, handed over through tracker states (#168)

stack-build and stack-close carry disable-model-invocation, so each phase change is
the operator's slash command; no skill offers a mid-session /model or /effort switch.
Mode rules and the phase table move to instructions/dev/stack-mode.md, publish and CI
waiting to instructions/dev/publish-and-ci.md, the ready definition to issue-tracking.md.

Files changed:
- AGENTS.md
- CHANGES.md
- DEVELOPMENT.md
- README.md
- VERSION
- docs/model-and-effort-selection.md
- instructions/CONTRACT.md
- instructions/dev/commonplace-kb.md
- instructions/dev/dev-setup.md
- instructions/dev/doc-pull-through.md
- instructions/dev/issue-tracking.md
- instructions/dev/publish-and-ci.md
- instructions/dev/stack-build/SKILL.md
- instructions/dev/stack-close/SKILL.md
- instructions/dev/stack-dev/SKILL.md
- instructions/dev/stack-mode.md
- instructions/dev/testing-conventions.md
- instructions/dev/version-parts.md
- tools/chemenu/commands/git_publish.py
- tools/chemenu/tests/test_git_publish.py
- tools/chemenu/tests/test_instructions_cmd.py

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SnAJ7Z3CpVD3PRbN73QtU2

docs/model-and-effort-selection.md

@@ -14,11 +14,23 @@ one more round. Work behind nothing but a session reading prose does not surface
 ships, and stays until someone happens to notice. That asymmetry, not task size, is what the
 phase guide below is built on.
 
+<!-- wikitool:toc -->
+## Contents
+
+- [Phase guide](#phase-guide)
+- [Model per session, not per phase](#model-per-session-not-per-phase)
+- [Subagent models](#subagent-models)
+- [`/code-review` effort](#code-review-effort)
+- [When it's unclear](#when-its-unclear)
+- [Scope](#scope)
+<!-- /wikitool:toc -->
+
 ## Phase guide
 
 <!-- dist:strip-start -->
-This repo's own stack-development work splits the axis into three phases, one per switch point
-in its `stack-dev`/`stack-close` skills:
+This repo's own stack-development work splits the axis into three phases, one skill each -
+`stack-dev` (design), `stack-build` (build) and `stack-close` (closing) - handed over through
+states in the issue tracker rather than inside one session:
 
 <!-- dist:strip-end -->
 | Phase / task | What would catch a mistake | Suggested model | Effort |
@@ -27,20 +39,48 @@ in its `stack-dev`/`stack-close` skills:
 | `wiki-lint` | `lint` itself is the check | Sonnet | default |
 | `wiki-ingest`, `wiki-manage`, judgment-heavy `wiki-query` | `lint` and `docs verify`, partly | Sonnet | high |
 | Stack dev: design, the version part, a boundary-crossing judgment | nothing mechanical | Opus | high |
-| Stack dev: code, tests, mechanical doc sync | `pytest`, `docs verify`, `instructions verify`, CI | Sonnet | high |
+| Stack dev: code, tests, mechanical doc sync, waiting for CI | `pytest`, `docs verify`, `instructions verify`, CI | Opus (open - see below) | high; medium for a small change |
 | Stack dev: closing an issue, `docs/` staleness, changelog prose | nothing, by construction | Opus | high |
 
-The middle stack-dev row is where the tokens are and where the checks are, so it is the one worth
-running cheaper. The two rows around it are short - minutes, not hours - so keeping them on the
-stronger model costs little and protects the only work in the session that fails silently.
+The two unchecked rows are short - minutes, not hours - so keeping them on the strongest model
+costs little and protects the only work that fails silently. The checked middle row is where the
+tokens are, which makes it the tempting one to run cheaper. It is also the row with the least
+settled answer: a build phase of this stack reads a lot of the tree, and a smaller context window
+does not hold it - it runs into compaction, which costs more than the cheaper model saves. Sonnet
+at high effort remains possible, but only as a session of its own (below). Which default the
+middle row should have is left to the record: every closed work package names the model, effort
+and session shape of each phase, and the table follows that evidence rather than the other way
+round.
 
 **Effort is the cheaper lever than the model.** A reduced effort level is what gives up
 multi-file consistency first, so `high` is a reasonable floor for anything touching more than one
-file or a contract; `default` suits a single-file mechanical edit with a test behind it.
+file or a contract; `default` or `medium` suits a small mechanical change with a test behind it.
 
-A session cannot switch its own model - that is the user's `/model` - so this table only pays off
-if someone offers the switch at the moment a phase changes, once, without turning it into a
-debate.
+## Model per session, not per phase
+
+A session cannot switch its own model - that is the user's `/model` - and it should not be asked
+to mid-flow either. Two reasons:
+
+- **A switch throws away the prompt cache.** A cache entry belongs to the model that wrote it,
+  so a new model starts the session's whole history from cold. The same holds for effort: changing it always invalidates
+  the cached message history - by far the largest part of a long session - and, on some models,
+  the tool and system prefix too (Anthropic's prompt-caching documentation lists effort and the
+  thinking configuration among what invalidates the cache). A switch from one effort to another
+  costs the same re-read of the whole session as a switch of model.
+- **An offered switch is rarely taken.** A sentence in the output at the moment a phase changes
+  is easy to read past - for the agent writing it and for the user reading it - and the session
+  just carries on in whatever it started as.
+
+So the choice is made once, when a session starts, and phases that want different models or
+effort levels are separated by a session boundary instead: `/clear`, then the next phase in a
+session started the right way. That is only cheap if the next phase does not depend on the
+previous session's context - which is why the handover has to live somewhere outside the session
+(an issue body, a page) and be kept current at fixed points, not reconstructed at the end.
+<!-- dist:strip-start -->
+In this repo the handover points are a ready issue body (design → build) and a green CI run with
+the body updated (build → closing); the two later skills can only be started by the user's slash
+command, so each phase change is a real stop at which that choice is made.
+<!-- dist:strip-end -->
 
 ## Subagent models
 
@@ -70,9 +110,10 @@ choice a session *can* make on its own:
   reflexively over-provisioning is a standing cost every session pays.
 - Not sure whether a phase is checked: treat it as unchecked - a needless Opus phase costs money
   once, an unchecked Sonnet phase can ship something nobody looks at again.
-- Mid-session and the phase changed but nobody switched: keep working - never block a publish or
-  an issue close on a model the session cannot change itself. Naming which model ran which phase
-  in the handover keeps the gap visible instead of silent.
+- The phase changed and the session runs on a model or effort the table would not pick: keep
+  working, and cut the session at the next handover rather than switching mid-flow - never block a
+  publish or an issue close on a choice the session cannot make itself. Naming which model and
+  effort ran which phase in the handover keeps the gap visible instead of silent.
 
 ## Scope