Coordinate migration from bundles to .py-owned bundle installs

[anth-volk]

Goal

Use this issue as the cross-repo index for retiring policyengine-bundles as the active bundle publication mechanism and moving official PolicyEngine bundles into policyengine.py.

The intended end state is:

• policyengine.py owns the official bundle version number.
• Users install official model-plus-data bundles with policyengine bundle install, e.g. uvx --from policyengine policyengine bundle install --venv .venv.
• Exact PolicyEngine-owned package versions and certified dataset versions are captured in .py bundle metadata and release assets.
• Downstream repos publish their own packages/artifacts normally; .py pins them when they should enter the official certified bundle.
• This repo remains archival unless historical validation scripts are useful for inspection or migration.

Cross-repo tracking issues

• Implement .py-owned pip-native PolicyEngine stacks policyengine.py#424
• Prepare policyengine-us for .py-owned stack releases policyengine-us#8684
• Prepare policyengine-uk for .py-owned stack releases policyengine-uk#1786
• Prepare policyengine-us-data for .py-owned stack releases policyengine-us-data#1172
• Prepare policyengine-uk-data for .py-owned stack releases policyengine-uk-data#434
• Prepare policyengine-core for .py-owned stack releases policyengine-core#503
• Use policyengine bundles for simulation worker installs policyengine-sim-api#571
• Audit Populace for new .py-owned stack install path microcosm#117

Implementation PRs

• Add .py-owned bundle installer and metadata policyengine.py#425
• Use policyengine bundle installs for simulation workers policyengine-sim-api#572
• Document PolicyEngine bundle install path policyengine-us#8685
• Remove policyengine-bundles contract CI policyengine-uk#1787
• Remove policyengine-bundles contract CI policyengine-us-data#1173
• Remove policyengine-bundles contract CI policyengine-core#504
• Archive active bundle publication #35
• Document PolicyEngine bundle boundary microcosm#164

Note: PolicyEngine/policyengine-uk-data is archived/read-only, so its old cleanup PR cannot currently be reopened or updated.

Work in this repo

• Disable active bundle publication workflows.
• Mark this repository archival in the README.
• Keep historical bundle records/scripts only where they are useful for inspection or migration.
• Do not require downstream repos to call into this repository when publishing model/data/package releases.

Acceptance criteria

• No active CI/CD path publishes new official PolicyEngine bundles from this repo.
• All active bundle publication documentation points to policyengine.py.
• Each linked repo has either removed old bundle dependencies, documented its boundary, or documented why no changes are possible.
• A maintainer can publish a new official bundle through a .py bundle metadata PR and normal .py release without orchestrating changes through this repo.

[anth-volk]

Opened the step 1 and step 2 draft PRs:

• Add .py-owned bundle installer and metadata policyengine.py#425
• Remove policyengine-bundles contract CI policyengine-core#504
• Document PolicyEngine bundle install path policyengine-us#8685
• Remove policyengine-bundles contract CI policyengine-uk#1787
• Remove policyengine-bundles contract CI policyengine-us-data#1173
• Remove bundle contract CI policyengine-uk-data#435

These cover the .py foundation work plus package-repo cleanup/documentation. The policyengine-bundles archival PR remains a later step after the .py stack path is accepted.