feat: add brownfield recipe for moving existing models into MetaObjects

CHANGELOG.md

@@ -15,6 +15,20 @@ here.**
 - **A `pydantic` extra on the Python package.** `pip install "metaobjects[pydantic]"` installs
   `pydantic>=2.8`, the minimum the generated Python models need, so an adopter can get the enforced
   floor with the package. The base install is unchanged.
+- **A recipe for moving existing models into MetaObjects without a rewrite.**
+  `docs/features/adopting-existing-models.md` takes Drizzle tables, Zod schemas and Pydantic models
+  from writing the model through retiring the old code, and proves the generated types match first.
+  The proof is a test file that `tsc` or pytest runs, so nothing reads your source and there is no
+  new command. `examples/brownfield/` holds a runnable project for each, and the recipe ships in
+  the `metaobjects-authoring` skill.
+
+### Fixed
+
+- **Python: an array field's `default` generated a string.** A model with
+  `field.string[]: { default: "{}" }` generated `tags: list[str] = '{}'`, which fails on first use.
+  It now generates `tags: list[str] = []`, from the same four spellings TypeScript accepts, or
+  from a native YAML list such as `default: [a, b]`. A default Python cannot spell as a list is
+  left to the database.
 
 ## [1.2.0] — 2026-10-30
 

agent-context/skills/metaobjects-authoring/SKILL.md

@@ -25,6 +25,7 @@ This file covers what almost every model needs. The topics below live in
 | File | Open it when |
 |---|---|
 | `references/adopting-existing-code.md` | working code or a populated schema already exists for what you are modeling |
+| `references/adopting-existing-models.md` | moving existing Zod, Drizzle or Pydantic models into the model: the step-by-step recipe, and how to prove the generated types match before the switch |
 | `references/choosing-a-shape.md` | no row of "Canonical form for common field needs" fits; you need the full UUID / timestamp / URI / `@autoSet` rule; or you are registering your own vocabulary |
 | `references/json-columns.md` | a JSON column beyond the ladder below: coverage per port, `field.map` at runtime, version caveats, bag → native array |
 | `references/identities-and-indexes.md` | declaring a primary, secondary or reference identity, an FK, or a non-unique `index.lookup` |

agent-context/skills/metaobjects-authoring/references/adopting-existing-code.md

@@ -13,6 +13,9 @@ redefining it. (The metadata is still the durable spine *going forward*; only th
 **The observable predicate:** does working code or a populated schema already exist for
 what you're modeling? If yes, you are in adoption mode and these rules apply.
 
+For Zod schemas, Drizzle tables or Pydantic models, `adopting-existing-models.md` is the
+step-by-step recipe: generate beside them, prove the types match, move one call site at a time.
+
 **Author metadata to match what the code ALREADY IS — not what you'd design fresh.**
 Read the existing code and schema *first*, then model to reproduce them:
 - The **native types the code uses** are the spec — model `field.uuid` when the code