Add context-pack.sh: one file per build step, brain wired into the build loop

[MendixMau]

What changed and why (one paragraph)

A helper used to put its own context together: read the brief, DESCRIBE entities one at a time, look for an example, guess the folder. Every lookup sends the whole conversation again. This PR makes that a single step. The module brief gets a ### Build steps table, with one row per dispatch listing what the step builds, what it reads, the example to copy, and the brain slice. The new project-bin/context-pack.sh <Module> <Step> turns that row into one file. The file holds the folder from the folder plan, the why from mxcli brain brief, the arch constraints, and live DESCRIBEs of every element the step reads, of the example, and of anything the step changes. The brief only names elements; the model describes them, and the pack is never committed. If a name is not in the model, the script exits 1 and lists it, so the problem shows up at dispatch time. iterative-build-loop.md now turns on mxcli brain at build start and gives every dispatch a pack. mdl-agent reads the pack first and records any why it learns with brain capture. Also included: a routing row (render-routing re-run), an install-manifest entry and a CHANGELOG line.

Field evidence

A/B on a scratch copy of a real app. Each arm built the same bulk-accept microflow with the same brief and the same brain, 3 helper runs per arm.

Arm	Tokens (mean)	Tool calls	Time
With pack	~130k	13	60 s
Without pack	~144k	21	101 s

• Result: about 9% fewer tokens, about 40% fewer tool calls and about 40% less time with the pack.
• Quality was the same in both arms. All 6 runs passed mxcli check, exec'd with 0 new mx errors and used the planned folder.
• The script exits 0 when the pack is complete, 2 for an unknown step, and 1 for a name that is not in the model. All three were verified.
• Probed on single-tree and two-tree (app/) layouts.
• Not yet run on Windows.

Checklist

• CHANGELOG line under ## Unreleased
• bin/render-routing.sh --check clean
• bin/check-scripts.sh, bin/check-portability.sh clean
• Leak guard clean; manual grep clean
• Windows / Git Bash run

🤖 Generated with Claude Code

https://claude.ai/code/session_01VJgWP5vEoAsNsJYqCDMGNw

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