ci: drop explicit Python install, use .python-version

Per PR #6 review: the workflow had `uv python install 3.12` while
.python-version pins 3.14, creating a CI/dev skew where CI could
end up testing a different interpreter than developers run locally.

Drop the explicit install step. uv sync auto-provisions whatever
.python-version pins, keeping CI and dev aligned. requires-python
">=3.12" in pyproject.toml still establishes 3.12 as the floor;
testing the floor specifically is a separate concern (matrix build)
we can add later if needed.

Author: chris-colinsky

.github/workflows/ci.yml

@@ -31,8 +31,9 @@ jobs:
         with:
           enable-cache: true
 
-      - name: Install Python
-        run: uv python install 3.12
+      # No explicit `uv python install` — `uv sync` auto-provisions the
+      # interpreter pinned in `.python-version`, so CI tests the same
+      # version developers run locally.
 
       - name: Sync deps
         run: uv sync --frozen