docs: expand MIGRATION.md to CNA depth

Closes #170.

Author: ulises-jeremias

docs/MIGRATION.md

@@ -1,7 +1,195 @@
-# Migration
+# Migration Guide
 
-From hand-rolled Cookiecutter/Copier flows:
+How to keep a scaffolded project aligned with improvements in
+`create-awesome-python-app` and
+[cpa-templates](https://github.com/Create-Python-App/cpa-templates).
 
-1. Map template repo to `templates.json` entry
-2. Add `cpa.config.json`
-3. Prefer `uv` lockfiles in generated projects
+## Why migration is hard
+
+`create-awesome-python-app` generates a **one-time snapshot** of a template plus
+extensions. After scaffolding, the CLI does not maintain a live link to your
+project. You own every file and dependency choice going forward.
+
+That is by design -- generated projects should be independent -- but it means
+updates require deliberate effort.
+
+## Strategy 1: Manual dependency updates
+
+1. Run `uv lock --upgrade` (or `uv lock --upgrade-package <name>`) in your
+    project to refresh pinned versions in `uv.lock`.
+2. Enable Dependabot if you used the `github-setup` extension (see its workflow
+    in cpa-templates).
+3. Compare your `pyproject.toml` scripts, dependency groups, and dev tools
+    with the current template in
+    [cpa-templates](https://github.com/Create-Python-App/cpa-templates/tree/main/templates).
+
+Use `uv tree --outdated` when available to spot stale direct dependencies.
+
+## Strategy 2: Diff against a fresh scaffold
+
+Scaffold a new project with the same template and extensions, then diff:
+
+```bash
+uvx create-awesome-python-app@latest my-project-new \
+  -t <template-slug> \
+  --addons <extension-slugs> \
+  --no-install \
+  --no-interactive
+
+diff -r my-project/ my-project-new/ \
+  --exclude=.venv \
+  --exclude=__pycache__ \
+  --exclude=.git \
+  --exclude=.pytest_cache \
+  --exclude=.ruff_cache \
+  --exclude=dist \
+  --exclude=build
+```
+
+Review differences in config files (`pyproject.toml`, `ruff.toml`, CI
+workflows, `.python-version`) and port changes selectively.
+
+## Strategy 3: Selective re-scaffolding
+
+For a single extension update:
+
+1. Scaffold a throwaway project with only that extension applied.
+2. Copy the extension-specific files into your existing project (e.g. Alembic
+    migrations, auth middleware, GitHub Actions workflows).
+3. Merge `pyproject.toml` dependency changes manually.
+
+## CNA to CPA renames
+
+If you are porting workflows, docs, or CI from
+[create-node-app](https://github.com/Create-Node-App/create-node-app), use the
+CPA equivalents:
+
+| CNA | CPA |
+|-----|-----|
+| `cna.config.json` | `cpa.config.json` |
+| `package.json` manifest | `pyproject.toml` |
+| `CNA_CACHE_DIR` | `CPA_CACHE_DIR` |
+| `CNA_REFRESH` | `CPA_REFRESH` |
+| `CNA_REFRESH_AFTER_HOURS` | `CPA_REFRESH_AFTER_HOURS` |
+| `CNA_NO_CATALOG_CACHE` | `CPA_NO_CATALOG_CACHE` |
+| `CNA_CATALOG_URL` | `CPA_CATALOG_URL` |
+| `CNA_STRICT_VERSION` | `CPA_STRICT_VERSION` |
+| `CNA_STRICT_REPRO` | `CPA_STRICT_REPRO` |
+| `CNA_SKIP_GIT` | `CPA_SKIP_GIT` |
+| `CNA_USER_AGENT` | `CPA_USER_AGENT` |
+| `~/.cache/cna` (default cache) | `~/.cache/cpa` (default cache) |
+
+CLI flags are the same shape (`--template` / `-t`, `--addons`, `--cache-dir`,
+`--refresh`, `--pin`, `--offline`, `--no-install`). See
+[cpa-vs-cna-config.md](./cpa-vs-cna-config.md) for manifest and tooling
+differences (npm vs `uv`, ESLint vs Ruff).
+
+## Template URL format and slug resolution
+
+The catalog lives in
+[cpa-templates `templates.json`](https://github.com/Create-Python-App/cpa-templates/blob/main/templates.json).
+Each entry has a `slug` and a `url`.
+
+**Slug resolution.** Pass a slug instead of a full URL and the CLI resolves it
+from the catalog:
+
+```bash
+create-awesome-python-app my-app -t fastapi-starter --addons github-setup
+```
+
+Run `--list-templates` or `--list-addons` to see available slugs. Invalid
+slugs fail with a hint to pass a full URL.
+
+**GitHub URLs with `?subdir=`.** Remote templates use the monorepo layout:
+
+```text
+https://github.com/Create-Python-App/cpa-templates?subdir=templates/fastapi-starter
+https://github.com/Create-Python-App/cpa-templates?subdir=extensions/github-setup
+```
+
+**Local `file://` URLs.** For forks, air-gapped work, or integration tests:
+
+```bash
+create-awesome-python-app my-app \
+  -t "file:///path/to/cpa-templates?subdir=templates/fastapi-starter" \
+  --no-install
+```
+
+**Pinning a revision.** Append `?ref=<sha-or-tag>` to the URL, or use
+`--pin <ref>` (equivalent to adding `ref=` to the template URL). With
+`CPA_STRICT_REPRO=1`, `ref` must be a full 40-character commit SHA.
+
+**Catalog override.** Point at a fork or fixture:
+
+```bash
+export CPA_CATALOG_URL="file:///path/to/templates.json"
+export CPA_NO_CATALOG_CACHE=1   # skip on-disk catalog cache
+```
+
+See [templates-json-schema.md](./templates-json-schema.md) for the full catalog
+shape.
+
+## CLI version changelog
+
+Track releases for template and extension changes:
+
+- [create-python-app releases](https://github.com/Create-Python-App/create-python-app/releases)
+- [cpa-templates releases](https://github.com/Create-Python-App/cpa-templates/releases)
+
+When upgrading the CLI:
+
+```bash
+# ephemeral (recommended)
+uvx create-awesome-python-app@latest --help
+
+# Homebrew / AUR / pipx installs
+brew upgrade create-awesome-python-app
+pipx upgrade create-awesome-python-app
+```
+
+Use `--strict-version` (or `CPA_STRICT_VERSION=1`) in CI to fail when the
+installed CLI is not the latest PyPI release.
+
+Manage the template cache:
+
+```bash
+create-awesome-python-app cache dir
+create-awesome-python-app cache list
+create-awesome-python-app cache update [id]
+create-awesome-python-app cache clean
+```
+
+## Adding extensions to an existing project
+
+There is no `cpa add-extension` command yet. To add an extension after the fact:
+
+1. Browse the extension in
+    [cpa-templates/extensions](https://github.com/Create-Python-App/cpa-templates/tree/main/extensions).
+2. Read the extension README for required files and dependencies.
+3. Scaffold a minimal project with that extension and copy the relevant files.
+4. Install matching dependencies from the extension's `pyproject.toml` (or run
+    `uv sync` in the throwaway project and mirror the dependency blocks).
+
+Use `--list-addons -t <template-slug>` to see extensions compatible with your
+base template.
+
+## Migrating from Cookiecutter or Copier
+
+If you are moving a hand-rolled Cookiecutter/Copier flow into the CPA
+ecosystem:
+
+1. Map your template repo to a `templates.json` entry (slug, name, category,
+    `url` with `?subdir=`).
+2. Add `cpa.config.json` at the template root (see
+    [cpa-config-schema.md](./cpa-config-schema.md)).
+3. Prefer `uv` lockfiles (`uv.lock`) in generated projects instead of
+    unpinned `requirements.txt` unless the template explicitly targets pip-only
+    workflows.
+4. Convert Jinja-style prompts to `customOptions` in `cpa.config.json` where
+    possible; use `--set key=value` for non-interactive runs.
+
+## Getting help
+
+- [Troubleshooting](./TROUBLESHOOTING.md)
+- [GitHub Issues](https://github.com/Create-Python-App/create-python-app/issues)
+- [Discussions](https://github.com/Create-Python-App/create-python-app/discussions)