ADR 0014: Host features reach extensions, and the API ships as stubs

Context #

Scopes were first a host-only mechanism. The owner wants extensions to be first-class users of every host feature, and wants people to build extensions with coding agents that can search the API.

Decision #

A host mechanism is not done until extensions can use it. Every new host feature ships with: the Starlark API and hook, a permission if it reaches outside the sandbox (UI-only features need none, and the docs say so), documentation and website pages, and updated .pyi stubs. The stubs live in stubs/zagros/ (one file per module), are embedded in the binary, and zagros ext init / zagros ext stubs write them with an AGENTS.md guide into a project. The site publishes them under stubs/. A Go test compares the runtime's predeclared names with the stubs in both directions, so they cannot drift.

Consequences #

Adding a host function without a stub fails CI. Dev files (zagros-stubs/, AGENTS.md, CLAUDE.md) are excluded from packed archives. The stubs describe a Python dialect for tooling only; extensions still run as Starlark.