Add PyPI publish workflow

.github/workflows/publish-to-pypi.yml

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+---
+name: PyPI
+on: push
+jobs:
+  build-n-publish:
+    name: Build and publish Python 🐍 distributions 📦 to PyPI
+    runs-on: ubuntu-latest
+    # Trusted publishing (OIDC): the publish step mints a short-lived token from
+    # PyPI instead of using a stored API token, so `id-token: write` is required.
+    permissions:
+      id-token: write
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          # hatch-vcs derives the version from git tags, so the full history
+          # (including tags) must be fetched.
+          fetch-depth: 0
+      - name: Set up Python 3.12
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.12'
+      - name: Install pypa/build
+        run: >-
+          python -m
+          pip install
+          build
+          --user
+      - name: Build a binary wheel and a source tarball
+        run: >-
+          python -m
+          build
+          --sdist
+          --wheel
+          --outdir dist/
+      - name: Publish distribution 📦 to PyPI
+        if: startsWith(github.ref, 'refs/tags')
+        uses: pypa/gh-action-pypi-publish@release/v1

CHANGES.md

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+# Changes
+
+This is a record of all past skillmodels releases and what went into them in reverse
+chronological order. We follow [semantic versioning](https://semver.org/) and all
+releases are available on [PyPI](https://pypi.org/project/skillmodels/).
+
+## 0.1
+
+- Add the Antweiler–Freyberger (AF) sequential MLE and the Attanasio–Meghir–Nix (AMN)
+  estimators alongside the existing CHS Kalman filter, with a shared control-function
+  correction, a public measurement-family interface on `ModelSpec`, and the AF
+  source/destination calendar adapter.
+- Add a GitHub Action to build and publish the package to PyPI on tagged releases.